INTRODUCTION: There are few studies on the potential benefits of green space quantity and quality for child well-being. The authors hypothesized that more and better quality residential green space would be favorable for well-being and that these associations could be subject to effect modification across childhood. METHOD: Multilevel linear regression adjusted for demographic and socioeconomic confounders was used to track change in well-being (Strengths and Difficulties Questionnaire Total Difficulties Score and ‘internalizing’ and ‘externalizing’ subscales) across five separate occasions among a cohort of 4,968 Australian children aged 4-5 years beginning in 2004. Well-being was assessed with respect to objectively measured green space quantity and parent-reported green space quality, with interaction terms fitted with age to assess for potential effect modification. Data were analyzed in 2015. RESULTS: Non-linear dose-response associations were observed for favorable well-being in relation to larger quantities of green space and also green space judged to be higher in quality. Favorable perceptions of green space quality were associated with larger quantities of green space regardless of neighborhood socioeconomic circumstances. Benefits for well-being appeared to top out at 21%-40% green space coverage and were reasonably consistent across childhood. Inequality in well-being (especially the internalizing subscale) emerged as children aged in relation to green space quality. CONCLUSIONS: Approximately 21%-40% of residential land-use allocated to green space may be an optimal amount for promoting child well-being, but the quality of this green space increases in importance as children age.