Purpose: To assess trends in South Australian children’s physical activity between 1985 and 2003 and 2013 in the contexts of school active transport, organized sport, physical education, and school break times. Method: The 1985 Australian Health and Fitness Survey was administered in South Australian schools in 2004 (8 schools) and 2013 (9 schools) and was demographically matched to the 10 South Australian schools in the Australian Health and Fitness Survey. Ordinal logistic modeling was used to identify trends, stratified by older children (10–12 y) and early adolescent (13–15 y) subgroups. Results: School active transport generally declined for males and older female children (19 and 20 percentage points per decade, respectively), whereas early adolescent females increased in walking to school (11 percentage points per decade). School sport participation declined for early adolescents, and club sport participation declined in early adolescent females (23 percentage points per decade). Moderate to vigorous physical activity participation during school break times (particularly lunchtime) generally declined (16–26 percentage points per decade). Observed changes in most contexts occurred between 2004 and 2013. Conclusion: There was evidence of context-specific declines in children’s physical activity participation over this time span. Early adolescent females were the subgroup of most concern, experiencing declines in most contexts.