Objective:​ Osteoporosis is a global health issue, and modifiable behavioural factors need to be identified in childhood to reduce the risk of osteoporosis in later life. The aim of this study was to investigate the influence of diet and physical activity on bone density of children aged 5–7 years participating in the Belfast Hyperglycaemia and Adverse Pregnancy Outcome (HAPO) Family study. Design and methods:​ Pregnant women were recruited to the Belfast centre of the HAPO study at 24–32 weeks gestation. Offspring were followed up at 5–7 years as part of the Belfast HAPO Family Study. Heel bone mineral density (BMD) and bone mineral apparent density (BMAD) were measured and calculated, respectively. Physical activity in the offspring was measured by accelerometery and dietary intakes were measured using a 4-day food diary. Results:​ Results from 793 offspring were analysed. Mean age of the offspring ± standard deviation was 6.4 ± 0.5 years. A mean of 48.3 ± 22.4 min each day was spent in moderate to vigorous physical activity (MVPA). Median (interquartile range) dietary calcium and vitamin D intakes were 844 (662–1073) mg/day and 1.7 (1.1–2.5) μg/day, respectively. Neither dietary vitamin D nor calcium intakes were significantly associated with offspring heel BMD or BMAD in multiple regression. However, controlling for confounders, a 30-min greater MVPA was associated with significantly larger heel BMD (0.018 g/cm² in boys and 0.010 g/cm² in girls) and BMAD (0.005 g/cm³ in boys and 0.003 g/cm³ in girls). Conclusion:​ Physical activity was associated with better BMD and BMAD in 5–7-year-old children. Dietary calcium and vitamin D were not predictive of BMD and BMAD.