Objective: To examine the effect of the intervention implemented in the ToyBox study on changes observed in age and sex specific BMI percentile and investigate the role of perinatal factors, parental perceptions and characteristics on this change. Design: A multicomponent, kindergarten-based, family-involved intervention with a cluster-randomized design. A standardized protocol was used to measure children’s body weight and height. Information was also collected from parents/caregivers via the use of validated questionnaires. Linear mixed effect models with random intercept for country, socioeconomic status and school were used. Setting: Selected preschools within the provinces of Oost-Flanders and West-Flanders (Belgium), Varna (Bulgaria), Bavaria (Germany), Attica (Greece), Mazowieckie (Poland) and Zaragoza (Spain). Participants: A sample of 6,268 pre-schoolers aged 3.5-5.5 (51.9% boys). Results: There was no intervention effect on the change in children’s BMI percentile. However, parents’ underestimation of their children’s actual weight status, parental overweight and mothers’ pre-pregnancy overweight/obesity were found to be significantly and independently associated with increases in children’s BMI percentile in multivariate modelling. Conclusion: Before or as part of the implementation of any childhood obesity intervention initiative, it is important to assist parents/caregivers to correctly perceive their own and their children’s weight status. Recognition of excessive weight by parents/caregivers can increase their readiness to change and as such facilitate higher adherence to favourable behavioural changes within the family.