What is available to families in the neighbourhood food environment is often assumed to shape children’s diets, yet evidence from quantitative studies remains inconsistent (1,2). To explore these discrepancies, we conducted a sequential mixed-methods study in Rotterdam, the Netherlands. In the quantitative phase, we analysed data from n = 3739 children (age 8 years) in The Generation R Study to examine cross-sectional associations between residential food environment exposure and child diet. Geographic Information System (GIS) measures within 400 m of each home captured: (i) absolute fast-food outlet count, (ii) relative fast-food proportion, and (iii) an overall neighbourhood healthiness score. Diet quality score (0–10) and non-core food intake (g/day) were derived from a validated parent-reported food frequency questionnaire. Generalised linear models adjusted for sociodemographic variables and tested moderation by maternal education. Unexpectedly, each additional fast-food outlet was associated with slightly higher diet quality (+0.012 points, 95% CI: 0.002, 0.022), with stronger effects in children of highly-educated mothers (+0.021 points, 95% CI: 0.008, 0.034). A higher neighbourhood healthiness score was associated with lower diet quality (−0.040 points, 95% CI: −0.073, −0.006). No associations were observed for relative fast-food exposure or non-core food intake. To interpret these counter-intuitive findings, the qualitative phase explored lived experiences of the neighbourhood food environment among n = 15 parents of children aged 8–10 years in socioeconomically diverse Rotterdam neighbourhoods. Semi-structured interviews were analysed using grounded theory. Parents’ engagement with their food environment was shaped not only by the number and type of nearby outlets (availability), but also by perceptions of accessibility (transport, time, mobility patterns) and acceptability (cost, quality, cultural relevance, preferences). Families living in areas with poor availability, accessibility, or acceptability of food outlets shopped beyond their immediate neighbourhood, using preferred outlets along commuting routes or online delivery to extend their food environment. Fast-food outlets, although abundant, were frequently dismissed as irrelevant to family diets due to health concerns or cultural mismatch. Integrating both phases, the “protective” association between fast-food density and diet quality in higher socioeconomic families likely reflects selective engagement and resource-enabled mobility. The inverse association between neighbourhood healthiness score and child diet quality may be confounded by the role of supermarkets and other “healthy” outlets as major sources of less healthy products. As such, this study shows that objective proximity-based measures capture only part of the food environment’s influence. By combining large-scale GIS analysis with in-depth qualitative insights, this research provides a rare, multidimensional perspective on how families navigate complex, dynamic foodscapes shaped by socioeconomic resources, mobility patterns, day-to-day context, and personal values. Effective policies to improve child diet quality must combine environmental planning with strategies that address the realities of how families perceive and use their food environments.