Understanding influences on children's physical activity and how these vary by activity and subgroup, such as age and sex of the child, is important for informing the development of effective and targeted interventions. Two cohort studies were conducted across socioeconomic areas of Melbourne, Australia, between 2001 and 2008 among a combined sample of more than 2,700 children aged 5-6 years and 10-12 years at baseline. Data were collected via surveys, and children wore the Actigraph accelerometer for 8 days. Five individual, 10 social, and 17 physical environmental factors were significantly associated with children's physical activity. Patterns of association varied according to the age and sex of the child and also according to the type of activity. These studies provide some insights into the various levels of influence on children's physical activity. More longitudinal and intervention research is needed to better understand the mechanisms of change in children's physical activity behaviour.