Treatment programs for childhood overweight and obesity have highlighted the importance of the family in treatment. Considering this, it is surprising that few studies have examined the role of family factors in the development of childhood overweight and obesity. The objective of this study was to examine which family and maternal factors predict increases in weight in boys and girls during middle to late childhood. This study used longitudinal data from the childhood growth and development (GAD) Study. The GAD Study involved 286 healthy weight, overweight and obese children, aged 6–13 years at baseline, who completed baseline, 1-year follow-up and 2-year follow-up assessments. Overweight/obese children were recruited from clinical and community settings. A broad range of maternal and family factors were assessed. Linear mixed models were used to identify which factors predicted child Body Mass Index (BMI) z-scores over time. For community-based children, maternal BMI and single-parent family structure were significant longitudinal predictors of child BMI z-scores. For the clinical participants, low family income was the only significant multivariate predictor of child BMI z-scores. The strong association between child BMI, maternal BMI and family structure confirms the need to target prevention and intervention efforts for childhood overweight and obesity towards families with overweight parents, particularly single-parent families.