**2. Methods**

## *2.1. Search Strategy*

The present systematic review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [50]. Relevant articles, indexed in three scientific databases (i.e., PsycInfo, Pubmed, and Web of Science), were searched. Further studies were identified through by-hand searches of the reference lists of the included articles. The investigation was conducted in June 2022, and the search included all original research articles published post-1968.

The exact search term combinations were: (["happi \*" OR "happy" OR "positive affect \*" OR "positive emotions" OR "subjective well-being" OR "subjective wellbeing" OR "well-being" OR "wellbeing" OR "life satisfaction" OR "satisfaction with life"] AND ["family funct \*" OR "family conflict" OR "family cohesion" OR "family communication" OR "family flexibility" OR "family problem-solving" OR "family problem solving" OR "family satisfaction" OR "family relation\*"] AND ["toddler \*" OR "infant \*" OR "child \*" "pre-schooler \*" OR "preschooler \*" OR "pre-adoles \*" OR "preadoles \*" OR "adolesc \*" OR "student \*" OR "pupil \*"]).

#### *2.2. Study Screening Selection*

Two reviewers independently selected abstracts, excluding articles that did not meet the selection criteria. Age and language filters were applied to the various databases to limit the search to studies reported in only English, French, Spanish, Italian, Portuguese, and German. Since the review focused on childhood and adolescence, studies involving participants over 18 years old were excluded. Specifically, only original research articles published in scientific journals were included in the review. Furthermore, only scientific studies using mixed or quantitative methodology were selected, while no studies involving clinical samples were included. Pure qualitative studies, books, and book chapters were excluded. No reviews examining the association between children's and adolescents' happiness and family functioning were found.

Moreover, to be considered for inclusion, studies had to assess both happiness and family functioning. Studies with a single measure evaluating the two variables as subdimensions (i.e., general life satisfaction and family satisfaction) were excluded. Only studies reporting associations between happiness and family functioning, or the effects of family functioning on children's happiness, were included. When the results appeared vague, the researchers contacted the authors (*n* = 50) to clarify their methodology and results (*n* = 8 responded). In the absence of a response, the relevant studies were excluded. Figure 1 displays the PRISMA flowchart of the systematic review process.

#### *2.3. Data Extraction*

The following information was independently extracted using a structured template by two reviewers: author(s), year of publication, country, study design, participant age and gender, sample size, measures of happiness and family functioning, and main findings. Coding disagreements were resolved through discussion between the first two reviewers. The Cohen's kappa coefficient, calculated to assess inter-rater reliability, was 0.94, reflecting very high agreement. The third author resolved any discrepancies.

**Figure 1.** PRISMA flowchart of the study selection.
