Roles of Urban Green Spaces for Children in High-Density Metropolitan Areas during Pandemics: A Systematic Literature Review

Abstract

For children living in one of the 30 megacities around the world, green spaces may be far away or tokenistic, with growing accounts of youth exhibiting a variety of nature deficit disorders. Amidst increasing pressure for more high-density metropolitan areas (HDMAs), international advocacy agencies are calling for policy makers and service providers to ensure good quality urban green spaces (UGSs) for all children. Furthermore, these green spaces need to be purposefully designed to cater to children’s needs, which continue to evolve amidst extreme weather such as heat waves. Recently, pandemics have placed additional constraints on how far children can travel and their time spent outdoors, making the quality of this experience for children even more important. This systematic literature review explored the roles of HDMA-UGSs for children during pandemics. From 551 studies retrieved and curated using the PRISMA methodology, 40 papers were subsequently thematically analysed. Four roles were synthesised for HDMA-UGSs in supporting children’s experiences during pandemics, which also benefit children during ‘normal’ day-to-day life’: (1) sustaining growth and development, (2) bridging social stratifications, (3) encouraging self-agency, and (4) facilitating independent mobility. Thirteen principles were distilled to support decision making in enabling these outcomes in new-build and refurbishment projects. The results are of immediate use for decision makers who are responsible for HDMA-UGS planning, procurement, installation, and maintenance. The UGS role descriptions enable targeted design interventions for supporting children’s health and well-being, including during times of constrained outdoor time.

1. Introduction

Access to urban green space (UGS) is essential for children as they grow up, promoting mental and physical development [1,2]. For children living in high-density metropolitan areas (HDMAs), urban green spaces (UGSs) can be beneficial, offering possibilities for recreational activities, social interaction, and educational experiences [2]. The United Nations (UN) Sustainable Development Goals (UN-SDGs) also identify the importance of green space for making cities inclusive, safe, resilient, and sustainable, and they call for more accessible HDMA-UGSs for children [3] amidst the rapid rise of ‘mega-cities’, i.e., cities with more than 10 million people [4]. Children’s well-being is being called into question, considering the compounding effects of rapid urbanisation and disruptions such as heatwaves and pandemics which are resulting in extended ‘stay-at-home’ lockdown and remote schooling experiences [5,6].

By 2050, almost 70% of the world’s children will live in HDMAs [5], with the potential for significant behavioural and mental health issues associated with ‘nature-deficit disorder’ [2,7]. Alongside rapid urbanisation, research has revealed that children’s outdoor experiences are also impacted by limited time spent outdoors (i.e., ‘outdoor time’) due to urban heatwaves, and limited interactions with quality UGS due to biodiversity loss and wildlife habitat degradation [8]. Christensen et al. (2017) note that children in HDMAs may be particularly vulnerable to heat-related health issues, respiratory problems, and other health issues associated with poor air quality [9]. More recently, the COVID-19 pandemic has further limited travel distances and constrained outdoor time for children and their families [10]. According to Dinda et al. (2021), children’s access to and use of UGSs has diminished significantly, resulting in missed opportunities for developmental milestones [10]. Famutimi et al. (2022) also conclude that there is a significant threat of the pandemic to children and their families’ mental and physical health [11].

This literature review also contributes to the broader field of ‘nature-based solutions for urban sustainability’, considering how HDMA-UGSs in dense urban habitats can be better planned and implemented, towards improved outcomes for children. It builds on previous research that suggests a variety of approaches to address challenges associated with rapid urbanisation and constraints on outdoor time. Fang et al. (2023) note that this field is still in its emerging phase, requiring more knowledge synthesis [12]. Lafortezza and Sanesi (2019) have explored the application of nature-based solutions in urban planning [13]. They present an applicable framework for local policymakers and service providers, which aims to enhance climate resilience and promote urban regeneration. Policymakers and service providers have a crucial role in developing and maintaining UGSs to provide children in HDMA environments (‘HDMA-children’) with outdoor environments that adapt to support their growth and well-being [14] amidst climate change and pandemic disruptions [15]. Quality assurance is also important, with calls for standardised evaluations of UGSs to support ‘early child education’ and ‘child-friendly cities’, as well as to inform relevant interventions [16,17,18]. Four studies have recently focused on how UGSs in high-density or high-rise urban environments addresses children’s well-being [1,19,20,21]. Flouri et al. (2014) demonstrated the positive impact of UGSs on children’s development, physically and mentally [17]. Dinda et al. (2021) evaluated the opportunities and challenges children face in accessing UGSs [10]. Agarwal et al. (2021) studied the need for standardised evaluations of UGSs to support early child education and child-friendly cities, as well as inform relevant interventions [16].

Within the context of rapid urbanisation, periods of restricted movement (i.e., confinement to the immediate local area) caused by the COVID-19 pandemic and extreme weather events associated with climate change, the authors of this review asked, ‘How are HDMA-UGSs supporting children’s physical and psychological development and well-being when outdoor time is being constrained?’.

A systematic review of the literature was undertaken, focusing on pandemics, with the assumption that supporting children’s experiences during pandemics would also benefit children during ‘normal’ day-to-day life. In framing the theoretical lens for the study, this research drew on ecological systems theory. The analysis spanned more than three decades of literature to synthesise four roles that HDMA-UGS play, and several principles that can support decision-making during the planning, design, installation, and maintenance. Examining the functions of HDMA-UGS in times of the COVID-19 pandemic presents an opportunity for policymakers and service providers to enhance children’s well-being in situations of restricted movement, with benefits also for contemporary living. The findings provide evidence that can be used to improve HDMA-UGS-related policies and design guidelines for mega-cities, enhancing socio-economic conditions, alleviating urban heat, and encouraging biodiversity.

2. Materials and Methods

This systematic literature review adopted a mixed search strategy that followed the PRISMA guidelines, including automatic searches of electronic databases, and manual searches of conferences and journal proceedings. The process is described in the following sub-sections. The PRISMA checklist is available in the Supplementary Material (See Supplementary Materials Table S2).

2.1. Terms and Definitions

Terms relating to this review vary globally, influenced by factors including language and translation of phrases, professional discipline, urban planning context and cultural preferences [22]. For this research, we have defined several key terms, as follows.

‘Service providers’ involved in providing HDMA-UGSs include professional stakeholders, spanning the lifecycle of planning, design, construction, and maintenance of these spaces. For this review, policymakers include, for example, government officers such as park managers, health department officers, and local government partners. Service providers include, for example, landscape architects, architects, and urban planners.

A clear definition of ‘children’ by age is important for understanding how various interventions and policies impact their well-being and development [18,23,24]. In this review, children were defined as individuals under 18 years of age, in order to consider all development phases [25]. The review included all ethnicities, genders, and nationalities.

The review focused on ‘high-density metropolitan areas’ (HDMAs) worldwide, describing the concentration of people and buildings in an area. High-density areas vary, based on regional and contextual factors, with no ‘standard’ internationally. In this research, high-density refers to the concentration of people, housing units, or structures in a given area [26,27]. In the literature the term can range from referring to a population density exceeding 1000 people per square kilometre (in Australia), to more than 10,000 people per square kilometre (in China). Within HDMAs, ‘high-rise’ (i.e., a vertical density descriptor) refers to tall buildings with at least 12 or 13 stories, noting that in the literature the term can refer to ‘exceeding 25 m or eight stories’ (in Australia) to being ‘only buildings over 100 m high’ (in China).

An ‘urban green space’ (UGS) can span features such as pocket parks, parklands, playgrounds, community gardens and informal green spaces [28,29]. Some studies also refer to including in situ examples: balcony, indoor courtyard, patio, indoor playground, vacant lands or lots and patches of spontaneous vegetation [30,31]. For this review, three UGS types were considered, namely ‘parks’, ‘gardens’ and ‘playgrounds’. The decision to focus on these three types of UGSs was based on their frequency of use in the papers screened. Of the 40 papers retrieved for screening, more than half of the papers used ‘parks’ terminology, half of the papers used the language of ‘playgrounds’ and over one-third used ‘gardens’. One fourth of the papers used both or all terms.

Within this context, ‘parks’ are referred to as sizeable public green spaces, overseen by the government or non-profit organisations, offering diverse recreational activities and amenities like sports fields, walking paths, and picnic zones [32]. ‘Gardens’ are referred to as smaller aesthetic spaces managed by individuals or community groups, focusing on ornamental landscaping and passive leisure [33]. ‘Playgrounds’ are referred to as outdoor areas for children, equipped with swings, slides, and climbing apparatus, either standalone or within larger parks, typically administered by government or non-profit entities [34,35].

2.2. Search Strategy

The authors adopted a hybrid search strategy, given the consideration of children’s well-being and the potential for consideration by health sector professionals and researchers. The search subsequently comprised a conventional key-word search string in addition to the use of a medical search string that embedded standardised (coded) language references. The combinations of medical subject heading (MeSH) terms and additional keywords that are relevant to ‘children’, ‘green spaces’, and ‘high-density’, are summarised in

Table 1. MeSH terms, keywords and finalised string.

Key Words	Children	Green Spaces	High-Density
MeSH Terms (for MedLine search engine)	Child Tree Number(s): M01.060.406 MeSH Unique ID: D002648	Green Spaces Tree Number(s): J03.925.680. MeSH Unique ID: D000068316	High-density Tree Number(s): F01.145.875.281 MeSH Unique ID: D003441
Additional Keywords (for other search engines)	Children, Kid	Parks (s), Playgrounds; Gardens; Greenery Grounds	Crowded; Populated; Metropolitan; High-density; Urban; Urbanisation; Urban sprawl; urban fortressing; segregation
	Pandemic: SARS; Pandemic; COVID-19; Coronavirus; fever; HIVI; Bird flu; influenza; HIV; H5N1 virus; Bird flu; H1N1; Swine Flu; the plague; Anthrax; Cholera; Leptospirosis.
	Mobility: Movement; Confined; Limited; Confinement; Restricted; Restriction

. The inclusion of ‘high-density’ related keywords in the literature search was driven by the research aim to focus on dense metropolitan areas. Within the land-use types occurring in cities, many researchers have discussed the challenges of sufficient green space in the most built-up (or dense) urban areas. With highly cited researchers such as Yao [36] and Güneralp [37] using the term ‘high-density’, this review adopted the same language.

The following single Boolean search string was used:

(TitleCombined:(Children; OR Child; OR Kid)) AND (TitleCombined:(High OR Density; OR Dense; OR Vertical; OR City OR Dwellers; OR Urbanization; OR Metropolitan; OR Inner OR City; OR High OR rise)) AND ((TitleCombined:(Green; OR Park; OR Green OR Space; OR Greenery; OR Garden; OR Playground)) OR (TitleCombined:(Pandemic; OR COVID-19; OR Coronavirus; OR fever; OR Bird OR flu; OR influenza; OR HIV; OR H5N1 OR virus))) NT (Fulltext:(Food; Allergy; Obesity; Mortality; Respiratory; Pregnant)).

The inclusion and exclusion criteria that were used to screen the extracted papers are summarised in

Table 2. Inclusion and exclusion criteria.

Category	Inclusion Criteria	Exclusion Criteria
Date	All dates of publications were considered
Language	Written in English or Chinese	Poor-quality English or Chinese
Type of publication	Peer-reviewed journal papers Conference proceedings and editorials, book and book chapters, reports	Literature reviews, letters, editorial and position papers If only the abstract is available Newspapers and trade papers
Research scope/themes	Discusses the association between children and urban green space in the contexts of high-density or high-rise living environments.	Urban green space is limited to in situ examples: balcony, indoor courtyard, patio, indoor playgrounds, etc.

. Conducting a literature review in both English and Chinese was considered beneficial for several reasons. Appreciating that English represents more than 90% of published research, prioritising and thoroughly analysing English literature was considered a fundamental requirement. Furthermore, the scale and pace of city development in China has resulted in significant exploration of planning challenges and opportunities, which may be limited in documentation to Chinese publications [38,39]. Considering the proficiency of the lead author in both English and Chinese, the research team decided to augment the English literature with Chinese literature. Where publications were in both languages, the research team provided the internal cross-check review. Where publications were in Chinese only, the lead author summarised the key content for the research team and the journal quality, in order to then undertake the internal cross-check of documents. Following this strategy, no Chinese publications remained in the list of included papers after filtering and screening.

The summary of database results for search-string sensitivity checking is presented in

Table 3. Summary of database results for string sensitivity checking.

Database Type	Research Result (without Timeframe)	10 Years	5 Years	1 Year
DOAJ	202	207	137	49
ERIC	6	1	-	-
JSTOR	4	1	1	1
PubMed	478	267	149	33
Science Direct	407	82	52	15
Scopus	539	317	200	60
Web of Science	457	274	169	49

, for the seven electronic databases searched. This table demonstrates the search results from seven databases and categorises the number of filtered papers within the time frames of ten years, five years, and the last year. To verify the selection process, the first author applied a test and retest approach after one month. A random sample of archived search results was also selected to compare these with previous searches, with no significant anomalies found.

2.3. Screening Process

The screening process was performed by the first author on the 501 extracted papers. First, publications found during the initial search were assessed for their suitability based upon analysis of their title and abstract, with irrelevant literature excluded [40]. Publications were then subjected to a more thorough analysis, as summarised in the PRISMA flow diagram in Figure 1.

The revised mixed methods assessment tool (MMAT) was used for quality assessment, which was conducted alongside data extraction [41]. The Reference ID with corresponding Article Checklist and MMAT assessment results are provided as Supplementary Materials (refer to Tables S3 and S4).

2.4. Thematic Analysis

Papers were thematically analysed using NVivo software, emulating the process previously conducted by researchers within the author team [42], drawing on Whittemore and Knafl’s (2005) synthesis framework [43].

A thematic analysis method was employed to identify emerging themes. The analysis began with a deductive or theory-driven coding system (a priori codes) using NVivo 11 software, and was expanded to include additional nodes (in vivo codes) derived from the emerging interview data. Axial coding was then used to reorganise the data into new categories (45 codes), identifying associations and links among the initial 204 codes [44]. Then three selective codes were created, categorising data into major categories and themes. The research team coded interview data, corroborating to reduce bias and validate the emergent themes [45,46]. The coding process followed documented processes for qualitative research [47,48]. The coding process and exemplar codes are depicted in Figure 2.

An inter-coder reliability rating of 80 percent was achieved, which is considered acceptable [49]. Four roles of HDMA-UGS were identified, along with associated hypotheses. For each of these roles, strategies have been proposed for policymakers and service providers to facilitate the integration of HDMA-UGS into urban planning and design practices.

2.5. Theoretical Framework

The ecological systems theory was used to frame the review, providing a lens through which to understand the dynamic interplay between children as individuals and their living environment [50]. Within this framing, four sub-theories were incorporated, namely urban morphology theory [51], play theory [52], cognitive development theory [53], and affordance theory [54].

The theory of urban morphology elucidates the physical characteristics and patterns of HDMA-UGSs. This theory provides insights into how the design and planning of HDMA-UGSs influence children’s interactions with their surroundings [51]. Play theory offers a psychological perspective, exploring the roles of HDMA-UGSs in children’s developmental, social, and well-being aspects associated with playful activities [52]. The theory of cognitive development contributes by examining children’s cognitive processes and developmental milestones influenced by exposure to HDMA-UGSs. The affordance theory enriches the framework by investigating the perceived opportunities and constraints that HDMA-UGSs afford children, revealing the reciprocal relationship between the environment and children’s behaviour [54].

By integrating ecological, morphological, psychological, and developmental dimensions, the theoretical framing facilitates examination of HDMA-UGS roles in multiple aspects of children’s lives within and outside pandemic periods. The theoretical framing is drawn upon for the findings in Section 3 and discussion in Section 4.1.

3. Results

This section presents the review findings considering the times and location of the studies, drivers, and focused types of HDMA-UGSs. A summary of the four major roles of HDMA-UGSs is presented, followed by a discussion of the policy and roles of service providers.

3.1. Descriptive Analysis

Of the 40 papers, 17 papers included quantitative studies, 12 comprised qualitative studies, and 11 used a mixed-method approach to the studies. In 26 of the papers, children were the key participants providing data, while 14 papers had non-representative samples: six focused on adults, and one focused on children. The following sub-sections highlight paper coverage in relation to timing, location and focus on UGS access.

3.1.1. Timing and Location of the Studies

The ‘global scan’ spanned more than 30 years. A total of 5 papers predated 2012, with 16 published from 2012 to 2021, and 19 published from 2021 onwards, indicating increasing activity in exploring the use of UGSs over the last decade. All 40 were English language papers, with 2 of them having a Chinese–English bilingual abstract.

Collectively, the 40 papers considered 331 cities in 21 countries across 4 continents. Within Asia (304 cities), there was a focus on China (298 cities), Indonesia (4 cities), and Japan (2 cities). The remaining 23 papers considered cities in North America, Africa, and Europe. Furthermore, the analysis revealed a relative lack of UGS studies in cities in Oceania, with only Australia represented (a total of three papers, all focused on Melbourne). The coverage of papers is shown schematically in Figure 3.

3.1.2. Scoping and Drivers for the Studies

The systematic literature review included papers with and without a pandemic focus, to enable insights to be made regarding the role of HDMA-UGSs in situations of confined arrangements and physical distancing. This ability to compare experiences to explore the performance of UGSs in different contexts aligns with the ecological systems theoretical framing of this review [50]. From a holistic perspective, this encompasses the immediate microsystem and the broader macrosystem. Reviewing papers without a pandemic focus provides valuable insights into how HDMA-UGS characteristics influence children’s engagement in day-to-day living from social, cultural, and ecological perspectives. Using an urban morphology lens, conducting a review of papers without a pandemic context becomes advantageous for highlighting the significance of UGS physical features on children’s daily routines and activities. The review found only the COVID-19 pandemic was referenced in the literature.

In summary, papers pre COVID-19 pandemic enabled exploration of the ‘baseline’ role of HDMA-UGSs in children’s daily routines, addressing a variety of challenges such as adult dominance, social inequity, and urban heat. Papers from the COVID-19 pandemic period enabled insights to be made regarding any differences in the role of HMDA-UGSs, addressing access constraints such as limitations in travelling beyond the home, and limited time outside the home. Eleven of the papers focused on high-density living and the COVID-19 pandemic, with only five of them considering the COVID-19 pandemic in cities (two in China and three in Japan). This coverage of just 11 papers on the topic of HDMA-UGSs for children during the COVID-19 pandemic underscores the need for more discourse on children’s health, development, and mental well-being in the context of COVID-19 pandemic.

Thirteen papers included ‘children’s health’ or ‘well-being’ in the title or abstract, using the improvement in children’s physical or mental health as an overarching driver for HDMA-UGSs and the researchers’ investigations. Eight papers were about realising the goal of a ‘child-friendly city’. Seven papers focused on ‘children’s play’, five of them focused on ‘low socio-economic’ aspects and three emphasised ‘children’s agencies’. The other four individually focused on ‘public health’, ‘social interaction’, ‘intergenerational injustice’ and ‘cognitive functioning’. Together, these nine types of drivers highlight end-user well-being as a key motivation for the published research on HDMA-UGSs, alongside informing an evidence-based approach to HDMA-UGS planning.

3.1.3. Types of Green Spaces

Fifteen of the papers discussed the general types of ‘outdoor play spaces’ (nine), ‘urban nature’ (four) and ‘urban spaces’ (two). Twenty-one focused on formal HDMA-UGSs, including ‘playgrounds’ (two), ‘green infrastructure’ (two) and ‘parks’ (seventeen), including ‘urban parks’ (nine), ‘neighbouring parks’ (six) and ‘school parks’ (two). Four papers focused on ‘informal green spaces’ (two), ‘courtyards in high-rise apartments’ (one) and ‘indoor green spaces’ (one). The frequency of referred UGSs, with various types highlighted, emphasises the focus on formal HDMA-UGSs in published studies. Furthermore, these numbers also underscore the knowledge gaps concerning informal HDMA-UGSs.

Considering the descriptive analysis of timing, location, drivers, and specific types of UGSs, there appears to be a growing emphasis on children’s interactions with UGSs in HDMAs, with research being driven by the desire to improve children’s well-being, through an evidence-based approach to policy and planning. Of the 331 cities referred to in the literature, the cluster of studied Asian cities can provide a focus from which to learn [38,39]. However, there were few studies of cities considering HDMA-UGSs and the COVID-19 pandemic.

3.2. Thematic Analysis—HDMA-UGS Roles

Four roles were synthesised for children’s UGSs in mega-cities, relating to the following: sustaining growth and development; bridging social stratifications; encouraging self-agency; and facilitating independent mobility. The discussion of roles is presented in

Table 4. Synthesis of literature regarding the roles of urban green spaces (UGSs) for children living in high-density metropolitan areas (HDMAs).

Author/s		Pandemic/s of Focus	Type of Study	UGS Roles (‘E’—Evidenced; ‘R’—Recommended)
				Sustaining Growth and Development	Bridging Social Stratifications	Encouraging Self-Agency	Facilitating Independent Mobility
Friedman et al. (2022)	[55]	*Y	*Qual	E	R	R	R
Lanza et al. (2021)	[56]	*Y	*Quant	R	E	R	-
Yao et al. (2022)	[57]	*Y	*Quant	R	-	E	R
Rios et al. (2021)	[58]	*Y	*Qual	E	R	-	-
Yamazaki et al. (2021)	[59]	*Y	*Mixed	-	R	E	-
Geneletti et al. (2022)	[60]	*Y	*Quant	-	E	-	-
Tomikawa et al. (2021)	[61]	*Y	*Qual	-	E	-	-
Venter et al. (2020)	[62]	*Y	*Quant	R	-	-	-
Yuta and Ryo (2020)	[63]	*Y	*Quant	-	E	-	-
Moore et al. (2020)	[64]	*Y	*Quant	-	-	E	-
Shoari et al. (2020)	[65]	*Y	*Quant	E	-	-	-
Huang et al. (2006)	[24]	-	*Qual	R	E	R	R
Huang et al. (2020)	[66]	-	*Quant	R	E	R	R
Whitzman and Mizrachi (2012)	[67]	-	*Qual	R	E	R	E
Kyttä et al. (2012)	[68]	-	*Mixed	R	R	E	E
Agarwal et al. (2021)	[16]	-	*Qual	R	R	E	-
Aarts et al. (2012)	[69]	-	*Mixed	R	R	E	-
van der Burgt et al. (2013)	[70]	-	*Qual	R	R	E	-
Andrews et al. (2019)	[71]	-	*Qual	R	-	E	E
Zhang et al. (2022)	[72]	-	*Mixed	R	-	R	E
Mohareb et al. (2019)	[73]	-	*Mixed	E, R	-	R	-
Bhuyan (2022)	[74]	-	*Mixed	R	E	-	-
Agha (2019)	[23]	-	*Qual	R	-	E	-
Andrews and Warner (2020)	[75]	-	*Qual	R	-	E	-
Aggarwal (2001)	[76]	-	*Qual	R	-	-	E
Ristianti and Widjajanti (2020)	[77]	-	*Quant	-	-	R	E
Roemmich et al. (2006)	[78]	-	*Quant	-	E	-	R
Farley et al. (2007)	[79]	-	*Mixed	-	R	-	E
Yuniastuti et al. (2019)	[80]	-	*Mixed	R	-	-	-
Simoneti (2000)	[18]	-	*Qual	E	-	-	-
Prihantini and Kurniawati (2019)	[81]	-	*Qual	E	-	-	-
Nugroho (2021)	[82]	-	*Quant	R	-	-	-
Nordbø et al. (2019)	[83]	-	*Quant	E	-	-	-
Sikorska et al. (2020)	[84]	-	*Quant	-	E	-	-
Marquet et al. (2019)	[73]	-	*Quant	-	E	-	-
Karsten (2015)	[85]	-	*Qual	-	-	E	-
Wells (2000)	[86]	-	*Quant	-	-	E	-
Garau et al. (2019)	[87]	-	*Mixed	-	-	E	-
Dewi (2012)	[88]	-	*Quant	-	-	E	-
Sharghi et al. (2014)	[89]	-	*Qual	-	-	-	E

*Qual: qualitative; *Quant: quantitative. *Mixed: qualitative and quantitative; *Y: The paper is focused on the context of the pandemic(s).

for each research paper and discussed in the following sub-sections.

3.2.1. Sustaining Growth and Development

The literature discussed the importance of designing UGSs to sustain children’s growth and development needs, both physically and psychologically. Three main considerations were documented, related to designing for children’s age hierarchy, accommodating children’s evolving experiences during pandemics, and providing overall mental support.

Ten researchers discussed the need to design UGSs according to the age hierarchy of children who are anticipated to use the spaces [18,23,64]. For children under six, gardens are the preferred play settings, over parks [66,83,90]. Conversely, children aged seven to twelve years, particularly seven to nine years old, benefited from multifunctional HDMA-UGSs promoting social interaction and development [74,81,90]. Playgrounds, as discussed in two other studies, can more efficiently support children’s recreational and social activities, compared with other types of UGSs [56,68].

Six researchers discussed the need for UGSs to accommodate children’s evolving experiences during pandemics, impacted by policy and socioeconomic factors. For example, during the COVID-19 pandemic, gardens that were primarily available to affluent families were less impacted by lockdowns [65]. The functionality of parks and playgrounds (i.e., the main open spaces for communities) faced restrictions due to social distancing policies and closure requirements, to various degrees [62]. Although the park closure requirements caused a general decline in child participation, they still offered vigorous recreational activities when accessible [65,69]. When closed, children autonomously began to use informal spaces, such as ‘sidewalks’ and ‘garages’ [76]. With structured activities also reduced by lockdowns, both formal and informal HDMA-UGSs catered to children’s increased need for physical activity [64,65,76].

Nine researchers discussed the potential for UGSs to have positive impacts on releasing children’s emotional pressures and increasing their living satisfaction. Eight papers discussed the role of UGSs in addressing emotional needs during the COVID-19 pandemic in urban areas, including promoting emotional development [58,64,65,80], releasing emotional stressors such as fear of sickness, spread of the virus, and concerns about pollution [55,56], lowering levels of behavioural and emotional problems [57], and offering emotional support [72,89]. Although children expressed fewer positive emotions during the COVID-19 pandemic, UGSs were considered to provide children with an avenue for reconnection [89]. Five studies concluded that having ample UGSs in neighbourhoods can increase children’s satisfaction with life [70,71,72], including their perceived safety and satisfaction [24,70]. Conversely, when UGSs suited children’s preferences in high-density areas, they were concluded to be more content with their environment [72,80,91].

3.2.2. Bridging Social Stratifications

The literature discussed the importance of UGSs in bridging children’s social stratifications, referred to as the ‘children’s well-being divide’; the gap between children with access to UGS benefits and those without [60]. Factors such as gender, ethnicity, socioeconomic class, and age influence this divide, affecting children’s ‘behaviour patterns’, ‘action radius’, and ‘independence’ [68,78,80].

Research indicates that boys and girls benefit equally from UGSs [63,70]. However, while both genders find value in UGSs, their preferences and attitudes, including regarding infrastructure and outdoor activities, do vary. Gender is as a ‘key influencing factor’ [71,76,92]. Past studies have discussed possible factors that influence children’s engagement with HDMA-UGSs. While boys and girls may have different metabolic rates impacting their preferences, post puberty ethnicity also plays a part; Latin American students, for instance, are less physically active than their white counterparts, a disparity rooted in historic discrimination and health inequities [64].

Age and socioeconomic status further define this relationship. In Dhaka, younger children preferred playing close to home more than older children [68]. ‘Class’ is another determinant of UGS access. Affluent children tend to have varying preferences [68], attitudes, and frequency of use with respect to UGSs [92], contrasting with their less affluent peers who depend on public space due to housing constraints [71].

Previous research has found that social stratification restricts equal access to essential social resources and services for children, exacerbating social and environmental injustice, attributed to housing typologies [84,93]. Affluent children typically live in city-centre homes with private outdoor spaces, whereas less privileged children, often in public housing, have limited access and rely more on nearby public areas [71,83].

Two researchers have discussed the impacts of the pandemic in exacerbating resource constraints for low-income children [65,73]. With formal HDMA-UGSs restricted, five studies highlighted the role of informal UGSs in bridging children’s well-being divide [55,61,73,76,78,89,92]. Developing and enhancing these spaces can promote equality and social justice [55,61,73,76,89].

It is noted that previous studies in China and Japan have found that socioeconomic status did not influence children’s access to HDMA-UGSs [64,92]. The authors of these papers noted that in China, research limitations stemmed from inadequate consideration of economic, social, and political factors influenced by places, historical trajectories, and inaccurate geographical data units. Similarly, Japanese research encountered limitations due to postal survey rates [94].

3.2.3. Encouraging Self-Agency

The literature discussed the importance of designing UGSs to consider the role of UGSs in supporting children’s agency while under adult supervision. Children’s agency involved creating action, including questioning, opposing, and acting differently [95]. The review identified various factors that can influence changes in children’s cognitive performance and be constraints on agency development [77,85].

The research indicates that parents can influence their children’s use and perception of UGSs through various means such as encouragement, cooperation, control, and monitoring [70,76,83,91]. This relevant parenting ideology is associated with factors such as education, marital status, and age [76,83,91]. Encouragement and cooperation of parents can enhance children’s perceived affordance of UGSs. However, their control and surveillance also resulted in children’s reduced sense of privacy, territoriality, and agency [70,77,83,85].

Family gentrification and parenting culture are responsible for the lack of engagement of middle-class children with UGSs [23,67,70,88]. Under social pressures, affluent children’s lifestyles are highly scheduled, and intensely institutionalised and micro-organised. However, two papers identified how children are also embracing their middle-class lifestyle to shape their values in life and express their desire for the culture and affluent lifestyles of the middle-class [23,80]. Another four papers argued that the negative impact is undeniable, including children who have expressed fears about the dangers of unstructured play and outdoor environments, and who rejected the notion of autonomous exploration of local environments, even losing abilities when coping with unstructured time independently [70,71,88,96]. All of these caused changes in their cognitive performance, and can be constraints that prevent children from fully developing their agency.

Although parents have greater entitlement, knowledge, access to material resources, and the ability to enforce punishments than children, the range of entitlements can be expanded or contracted, based on children’s power efforts [87]. Children show their power by acting and managing lives by their agency [86]. Twelve studies demonstrated the positive impact of UGSs on children’s agency, including:

• Benefiting children by promoting their cognitive functioning, improving their understanding of urban living and citizenship, enhancing their entitlement to better facilities and services, and positioning them positively in adult society [56,57].
• Supporting children in practising reimagining and inventing space, thus strengthening their agency and beliefs in utilizing UGSs [23,59].
• Enhancing their agency and agency beliefs by encouraging them to create their own space and shape their own play experiences autonomously [23,71,81,85,96].

Once children were presented with opportunities and actively acknowledged them, these opportunities transformed into affordances for children to perceive. Three papers mentioned the concepts of ‘affordance’ and argued the importance of the affordances of UGSs for children’s well-being [64,65,76]. ‘Affordance’ was referred to in order to prove the role of UGSs in not only supporting children’s agency and enhancing their beliefs in using UGSs, but also implying children’s multiple approaches to benefit from UGSs, based on the enhanced affordance of the latter.

The final interesting findings were that that the child’s agency also influences the use of UGSs by their parents. Two studies showed that families with children were significantly more likely to use UGSs [97]. The presence of children can increase the need or desire of parents to participate in neighbourhood UGSs, services, and amenities [66].

3.2.4. Facilitating Independent Mobility

Five researchers discussed the importance of designing UGSs to promote children’s independent mobility within HDMAs. Independent mobility pertains to the autonomy of children to move within their neighbourhood or city without adult supervision [98]. Evaluation of this can involve metrics such as ‘territorial range’, denoting the distance children can venture from their residence without supervision [99], and ‘mobility licenses’, gauging parental views on their children’s potential activities and the monitoring carried out to ensure safety [70,79,100].

UGSs and relevant structures in children’s environments can expand their territorial range, increasing outdoor activities and travel distances [56,63]. Proximity to UGSs enables children’s autonomous experiences, promoting spontaneous practices and exploration [85]. In HDMAs, children exhibit high independent mobility and increased use of public transportation, particularly in densely populated and disadvantaged communities [56,57]. However, some studies suggest contrasting perspectives. One argument suggests that children in HDMAs may be less active due to restrictions imposed by their parents’ mobility licenses, while others propose the opposite [56,64,85]. One explanation for these conflicting results is the consideration of safety, which was overlooked in the previous two studies.

Safety concerns are a primary issue for parents [64,66,67,72,80,89,101]. Creating UGSs in neighbourhoods offering high levels of safety, walkability, and accessibility can potentially increase parents’ comfort in encouraging their children to play outdoors and promote their independent walking [64,66,67,82,101]. However, a separate paper argued that the number of mobility licenses did not significantly affect the proportion of green structures [56]. This conflicting result may be attributed to the limited methods used and the absence of family-specific socioeconomic variables in the collected data.

3.3. Thematic Analysis—Policy and Service Provider Roles

A thematic analysis of policy and service provider roles sought to shed light on the intricate dynamics and responsibilities that both bear in the context of the four UGS roles detailed above. The following sections discuss how policies are formulated and implemented, as well as how service providers interpret and execute these policies.

3.3.1. Policy Environment

Sixteen papers mentioned the policy environment, including the COVID-19 pandemic policy (three), housing-related policy (six), HDMA-UGS design-related policy (four), health policy (one) and economic policy (one). Among these mentioned polices, seven of them focused on the impact of policy, three focused on ‘children-centred’, five focused on ‘suggestions’ and two focused on ‘COVID-19′. The key polices mentioned in these 40 papers are listed in

Table 5. Key Policy Examples.

Key Policies Examples	Type of Policy	Reference
COVID-19-related policy; key policy measures to reduce the transmission of SAR-CoV-2 and protect public health.	COVID-19 Pandemic Policy	Shoari et al. (2020) [65]; Yao et al. (2022) [57]; Venter et al. (2020) [62]
The government incentive policy issued in 1984; policy levers; government policy on housing design; no explicit child-friendly policy on high-rise, private housing. Apartment guidelines (the new Victorian Apartment Guidelines); Australian policy on the design of high-rise dwellings; integrative planning practices and solutions.	Housing Policy	Whitzman and Mizrachi (2012) [67]; Nordbø et al. (2019) [83]; Aggarwal (2021) [76]; Andrew and Warner (2020) [75]; Andrews et al. (2018) [71]
Child-friendly-city policy; policies regarding the use of specific green areas, including parks, agricultural lands, and gardens; policy related to local spatial planning and traffic and transportation; common international standards and policy objectives for local green space planning (16 policy scenarios); spatial planning policy.	Design Policy	Dewi (2012) [88]; Yao et al., (2022) [57]; Aarts et al. (2012) [69]; Geneletti et al. (2022) [60]; Sikorska et al. (2020) [84]
Policy in relation to children’s health in Australia.	Health Policy	Andrews et al. (2018) [71]
Current planetary economic policy.	Economic Policy	Rios et al. (2021) [58]

.

While improving, the current policy environment is still not directed toward children and their families; for example, policies on high-rise building design [66], and policy interventions and regulations that can be adopted in an emergency [78]. The barriers include neglecting the diverse needs of children from different genders, age, or nationalities [92], undermining children’s agency [55] and uncoordinated service delivery, and inadequate implementation of a ‘child-friendly city’ [70]. Suggestions are also made, such as policy for facilitating the use of specific green areas, for example, agricultural lands [89].

3.3.2. Service Provider Roles

Going forward, HDMA-UGS improvements will require enhanced multisectoral professional contributions. Among the 40 papers, authors put forward various strategies to enable HDMA-UGS roles during the lifecycle of these spaces. To enable the first role of sustaining growth and development, the strategies include the enhancement of public participation [61], the adoption of a child-centred design [102], the emphasis on families’ needs [91], the adoption of technology into UGS use [83] and the development of both formal and informal HDMA-UGSs [73]. To enable the role of ‘bridging social stratifications’, authors put forward strategies for focusing on low-socioeconomic children [92], emphasising middle-class families’ needs [71], and balancing social injustice [55]. To enable the role of ‘encouraging self-agency’, possible strategies include the provision of a parent-satisfied environment [63] and providing a positive experience for children [56]. The possible strategies for enabling the role of ‘facilitating independent mobility’, include adoption of the technology [62], enhancement of parenting awareness [96], improvement of informal play spaces [76] and emphasis on the concept of affordance [86].

Table S1 (provided as ‘Supplementary Materials’) summarises the opportunities for service providers to consider in new UGS or refurbishment projects, to accommodate children’s needs during periods of confinement.

4. Discussion

The literature’s focus on four key roles for HDMA-UGSs is useful in curating current and future greenspace within mega-cities. As noted in the introduction to this review, within two decades most of the world’s children will experience HDMA-UGSs as their primary contact with nature [1,2,3,5,103]. Furthermore, the COVID-19 pandemic and other disruptions further limit children’s access to outdoor spaces, requiring close attention to be paid to the type and accessibility of HDMA-UGSs for children [104,105].

As concluded by 13 researchers across the 40 reviewed papers, an HDMA-UGS designed and constructed with these roles in consideration would support physical and emotional development during pandemics and during ‘normal’ day-to-day mega-city life. In the following sub-sections, we discuss the findings in relation to the benefits of these considerations for day-to-day mega-city living, and the role of service providers in catering for children’s experiences during the COVID-19 pandemic.

4.1. Adapting Service Provision to Cater for Periods of Restricted Movement

The following sub-sections discuss important actions towards optimising HDMA-UGSs, comprising the following: sufficiently integrating these spaces spatially; intentionally using UGSs for exploration and discovery; ensuring the UGSs are culturally and environmentally attuned to place; and catering for diversity in children’s age and ability.

4.1.1. Sufficient Geospatial Integration

The physical layout and design of urban, and community environments can allow children to interact with their surroundings effectively and seamlessly. Carins (2017) stated that UGSs can be discursively constituted as a site of reconnection for children with nature, recuperating the essence of childhood within the urban landscape [105]. However, given the past research, children are not autonomous either in the city or in neighbourhoods, and do not live their urban experiences in the same manner as adults [106]. Processes of privatisation, exclusion and security underlying the city space also shape the new urban geographies of child-unfriendly play [107]. Six papers discussed the importance of HDMA-UGSs in stimulating city children to re-claim and occupy the public spaces freely, initiatively, and independently. However, urban children are still disconnected from nature, plants, and the Earth, which is a long-term challenge for policy and service providers [108].

Given the analysis of case studies among 40 papers, the existing drawbacks of geospatial attributes for HDMA-UGSs can impede children from accessing and using these spaces. Whether these HDMA-UGSs provide sufficient free play chances for children is one of the essential criteria [109]. However, past studies argued that neither the children’s cities nor the public playgrounds can fully meet the benchmarks of the right to play [107]. Independent mobility is identified as an important contributor to children’s social health, by giving them opportunities to build and sustain their bonds with peers and develop their relationships with their neighbourhood and local community [110,111]. A decline in children’s independent mobility during recent decades is well documented in developed countries and also supports the premise that the disconnection exists [79,112,113,114].

Building upon the discussion of the roles of policy and service providers, through prioritising sufficient integration of UGSs spatially, service providers, especially urban planners and landscape designers, can improve children’s access to HDMA-UGSs. These spaces should be integrated into the broader urban or geographical context in a sufficient and meaningful approach. This finding further supplements the theory of urban morphology. Through this theoretical lens, HDMA-UGSs should not be isolated, but are connected to the surrounding neighbourhood, schools, residential communities, and other urban features that cater to urban children’s biophilic needs.

4.1.2. Designed for Exploration and Discovery

Policymakers and service providers can consider the safety of children’s needs, rather than completely focusing on their parents’ perspectives on safety. Three papers revealed how UGSs empower children’s agency, to live, play and explore within metropolitan areas under adults’ control and supervision, considering the variability of parents’ social status, education level, and schedules. HDMA-UGSs support children’s individual rights and awareness to use these spaces by stimulating their imagination, creativity and biophilic desire. However, as emphasised in the past research, most urban parents are inclined towards restricting children’s agency, which is caused by the difference between children’s and parents’ perceptions: this refers to the fact that parents tend to give more generalised explanations and to relate safety and risks to a general perception of society and its change [115].

Past studies have discussed how UGSs can promote children’s agency, but few of them have indicated how this agency can be adopted to assert their presence and space in opposition to adults’ dominance within the HMDAs. Past research examined the significance of safety in UGS design [116]. However, there was little discussion about what safety means for children. Most of the discussion was oriented around adults’ understanding and experience of making children ‘safer’, subjectively identified by parents and limiting children’s ‘mobility licenses’ [107,114]. However, given the theories of Jean Piaget’s cognitive development phases, children’s thinking is qualitatively different from that of adults. This disparity may result in safety designs which are based on adults’ understanding, not aligning with children’s expectations. Parents’ concerns for their children’s safety may be higher than actual risks [53]. The concerns can limit children’s agency and opportunities to explore the natural world. Considering the Play Theory by Lev Vygotsky, UGSs can serve to stimulate children’s initiatives and agency, remembering that children are not merely passive receivers, but also active contributors who can make an impact [23]. Without over-supervision from their parents, children can explore public spaces in journeys of discovery.

Discovery and exploration can also be enabled through ‘prospect’, ‘refuge’ and ‘peril’, as outlined by Niranjika (2022) and Rupprecht (2014), in the context of nature-loving cities, or ‘biophilic design’ [117,118]. ‘Prospect’ refers to the idea of having clear, unobstructed views of the surrounding environment, and ‘refuge’ is the concept of creating spaces that provide a sense of safety and security while still being in proximity to nature. In particular, ‘peril’, the introduction of controlled elements of danger into the built environment, creates experiences that evoke in children a sense of adventure, excitement, or curiosity. Policymakers and service providers can use these findings to design HDMA-UGSs for children’s independent, critical, and free exploration and discovery, further improving their well-being.

4.1.3. Place-Based, Environmentally Attuned Elements

Policymakers and service providers can incorporate place-based, environmentally attuned elements into urban design to enhance sustainability and community well-being. These elements are focused on the local adaption of geospatial attributes. Past research has discussed the existing knowledge regarding the significance and limitations of formal HDMA-UGSs, particularly gardens, parks, and playgrounds. Informal UGSs also play an increasingly important role in the urban fabric, and are emerging as a sub-discipline of urban environmental planning and recreation [119], However, less research has focused on informal UGSs such as the ‘vacant lot’, ‘spontaneous vegetation’, wildland’, ‘brownfield’, ‘wasteland’, ‘leftover space’ [120], ‘courtyards in high-rise apartments’, ‘community gardens’ and ‘recreational vacant lots’ mentioned in this literature, and their potential for city children’s well-being in the HDMA context. As Pincetl and Gearin (2005) suggest, the policy and service provider should use the UGS as a broader concept, as children also value informal and unmanaged areas for their free play and exploration [120]. The potential for manipulating environments, whether unmanaged or managed, should be acknowledged as an integral aspect of children’s play and should be met with understanding by administrators, who must address the diverse perspectives on places held by both adults and children [121,122].

Affordance theory suggests that the environment contains perceivable properties that directly inform children’s behaviours [123]. This theory underscores the active and dynamic nature of children’s interaction with their environments, and highlights the significance of recognising the perceived opportunities and limitations within the environment, which, in turn, influence children’s play, learning, and development [124]. Past studies have argued that children can find affordances for play in both formal, planned spaces and informal or unmanaged places [122]. The finding of this review further stated that informal spaces have the potential to elicit social and psychological benefits through enhancing their agency for city children. In this way, informal HDMA-UGSs can bridge the well-being divide among children caused by race, gender, and class, encouraging inclusive, sharing, and collaborative behaviours through informal and formal spaces [73,120]. Policymakers and service providers can use this finding to advocate for the preservation and creation of informal UGSs within HMDAs to address inequities and cultural sensitivity.

4.1.4. Catering for Diverse Ages and Abilities

Policymakers and service providers need to extend beyond mere alignment with the diverse lifestyles of caregivers, considering factors such as social status, educational background, and work–life schedules. They need to consider the profiles of children as the primary users, including their predictable behaviours and attitudes and the subsequent influence on their families.

These themes also clarify that ‘homogeneity’ can be an ignored weakness of the current UGS system in HDMAs. Some researchers discussed how individual characteristics of children, such as age, class, or ethnicity, influence their relationship with HMDA-UGSs, while few researchers focused on how UGSs can be tailored to accommodate the diverse profiles of children more effectively. Given the findings in this review, most current UGS systems have ‘mono’ features that ignore the shifting needs of children with continuous, dynamic, and long-term development. This weakness has had light shed on it during the COVID-19 pandemic period.

This finding can also supplement the ‘affordance theory’, to aid policymakers and service providers in better understanding the current UGS system in HDMAs, with the lack of a dynamic and iterative nature. As children grow, their needs change, necessitating the adaptation of UGSs to meet their evolving requirements. There is a lack of research focusing on children from diverse backgrounds and specific features. For example, most research has grouped children into broad age ranges, without considering the distinct needs and preferences of different age groups, or concentrating on the broader socio-environmental context rather than specific living spaces, which can result in the potential conflation of factors [76,125]. Future research should adopt a ‘child-centred’ methodology, to better realise the specific dynamics and nuances of children’s experiences in different urban living contexts.

4.2. Applying the Learning from Pandemics to Day-to-Day Living

The review findings highlight the additional constraints imposed through the COVID-19 pandemic on the lived experiences of children living in HDMAs, namely limitations in travel, and limitations in time spent outside. The consequences of these constraints are documented as far-reaching, spanning children’s mental and physical well-being, family relationships, friend networks and social development. In addition to the clear mental and physical well-being benefits from UGSs documented across the papers, during the COVID-19 pandemic HDMA-UGSs also supported children in maintaining their health during lockdowns, and provided a ‘social bridge’ to family and community. In the papers’ examples, HDMA-UGSs performed significant practical roles in supporting children’s well-being, towards ensuring equitable access for children to quality interactions with nature.

The need for improved access to HDMA-UGSs is reflected in the 15 researchers in the studied papers, who noted the importance of access to nature being an overarching driver for HDMA-UGSs. From this literature, there are several significant end-user benefits when service providers enable the roles discussed in the above section:

4.2.1. Improving Socio-Economic Factors

Environmental injustice still exists in HDMAs. The availability and accessibility of UGSs are significantly and positively correlated with income and socio-economic status [126]. Children from affluent families have the privilege of accessing private UGSs to connect with nature, while also benefiting from a well-maintained, high-quality UGS system. In contrast, children from low-socioeconomic families often rely on services provided by public open spaces, which may carry the risk of low quality and limited accessibility. During any natural or human disaster period, children from the latter group are deprived of an equal opportunity to access nature and are exposed to more severe nature deficit disorder.

Given the findings of nineteen papers in this review, HDMA-UGSs play the role of bridging social stratification for children from low-socioeconomic-status families [65,73]. By implementing the strategies of developing both formal and informal HDMA-UGSs to improve the HDMA-UGS system, it is possible to improve the equity and inclusion of society [65]. This finding is aligned with past studies that have argued for the benefits of informal UGSs for children’s mental and behavioural health, living in deprived families [127]. Affirmative action on HDMA-UGS planning is required to redress the socioeconomic inequity in terms of access to this important public health resource [128].

4.2.2. Mitigating Urban Heat in High-Density Metropolitan Areas

The growing rate of urbanisation and alterations in natural land-use properties are causing many worldwide environmental issues such as urban heat islands (UHIs). Urban heat not only threatens children, but also society at large. Seven out of forty papers mentioned urban heat, discussing community well-being and mechanisms to address this vulnerability. The researchers also concluded that HDMA-UGSs were a major component in the urban development system, playing a vital role in mitigating the urban heat effects and improving children’s health [65].

This role is aligned with past studies’ arguments that various HDMA-UGS layouts in an urban area affected the surrounding atmospheric air temperature that contributes to a heat island effect [129]. The development of HDMA-UGSs is one of the most proposed measures for mitigation of UHIs and improvement in thermal comfort satisfaction, which causes lower cooling energy consumption and indirectly improves the urban air quality [130]. These findings emphasise the significance of considering and further exploring the scale dependence of the UGS cooling effect in mitigating urban heat [131]. These findings further prove that many HDMA-UGSs and green infrastructure functions are complementary to resiliency goals [132].

4.2.3. Promoting Biodiversity in Mega-Cities

A biodiverse urban environment benefits the health of all residents. Understanding the development process of urban green spaces and biodiversity conservation strategies in urban green spaces is vital for sustainable urban development [133]. However, the global decline in biodiversity because of urbanisation remains poorly understood [134]. The concept of urban biodiversity is still an ecological term frequently confused and interchangeably used with UGS [135].

Given the strategies that can be adopted in future HDMA-UGS development, the provision of an abundance of both formal and informal UGSs can serve as habitats for plant and animal species, to enhance local biodiversity. The findings of this review align with past research. HDMA-UGSs have been proven to be a refuge for urban biodiversity [133,136] and to play a key role in maintaining the socio-ecological memory of the urban system [135]. Vacant lots (a form of urban green spaces) are being repurposed for habitats for biodiversity [11]. The incorporation of people’s preferences, perceptions and values into biodiversity-sensitive urban designs further facilitates the conservation of urban biodiversity [135].

4.3. Theoretical Contribution and Suggestions

Given the discussion above, future research can focus on employing more longitudinal and cross-disciplinary methodologies, exploring the dynamic interplay between urban environment design and children’s development. This would build on the supportive evidence for urban morphology theory, play theory, cognitive development theory, and affordance theory presented in the previous section and illustrated in Figure 4.

To further explore the relationships among these theories and contribute to the development of effective urban planning strategies, spatial analysis can be undertaken to compare and contrast planning strategies and successes in similar HDMA habitats from around the world. Planning lessons learned from such comparative analysis can also benefit from integrating perspectives from other fields such as environmental psychology and education.

4.4. Limitations

During analysis, three limitations were observed to influence the findings of this review. Firstly, the review may have missed relevant sources that were not available online. Secondly, the exclusion of studies published in languages other than the one(s) in which the review was conducted may introduce language bias. Researchers could consider a more cross-cultural approach to gather relevant information from other cultures in the future. Lastly, although the research focused on children in densely urban areas, this was not consistently clarified in the examined studies.

To mitigate the limitation of potentially missing relevant sources, the research team considered conducting a more comprehensive search, which involves using multiple databases, libraries, and resources to ensure a more exhaustive literature review. The research team developed a clear and consistent inclusion- and exclusion-criteria document to mitigate the issue of inconsistent clarification regarding the focus on children in HDMAs. The research team also consistently applied these criteria during the review process. Future research could explore how findings vary across different contexts and populations, providing insights into the generalizability of existing research.

5. Conclusions

In conclusion, access to HDMA-UGSs is beneficial for the holistic development of children, providing opportunities for physical, mental, and socio-emotional growth [1,2]. However, the increasing challenges posed by mega-cities, rapid urbanisation, climate change, and recent disruptive events like the COVID-19 pandemic have highlighted the vulnerability of children’s well-being in HDMAs [4,5,10]. Addressing the roles of HDMA-UGSs during COVID-19 pandemics is important in contributing to the evolving field of ‘nature-based solutions’ for urban sustainability [12] and providing opportunities for decision makers to positively influence children’s well-being during times of constrained movement and, more generally, in 21st century living.

The analysis of 40 papers from more than 30 years of research into the COVID-19 pandemic and high-density metropolitan areas has highlighted the usefulness of UGSs in supporting the physical and psychological needs of children of different age groups during pandemics. UGS also promotes children’s independence and autonomy by enhancing their sense of privacy, territoriality, and entitlement to use space. This also in turn promotes independent mobility for children and safer mobility for parents. It has also highlighted the need for priority attention by multiple service providers to ensure quality HDMA-UGS outcomes. Key steps in enhancing HDMA-UGSs involve effectively integrating UGS spatially; purposefully utilizing UGSs for exploration and discovery; ensuring cultural and environmental alignment with the surroundings; and accommodating diversity in children’s ages and abilities.

Therefore, by exploring the roles of HDMA-UGSs during COVID-19 pandemic, this systematic review has highlighted the multifaceted contributions of UGSs to children’s physical and psychological well-being, in the context of contemporary urban living. The identified roles and principles can inform policies, design guidelines, and interventions for mega-cities, fostering socio-economic conditions, mitigating urban heat, and promoting biodiversity.