Although accumulating research suggests that acute physical exercise ameliorates cognitive function in adults, little is known about the effects of acute exercise on cognition during development. We assessed simple reaction and choice response times in 7- and 10-year-old boys (n = 36 per age group). Half of the children completed 30 min of aerobic exercise, whilst the other half watched television. Each child was tested immediately prior to and immediately following the intervention. Compared to the control group, the children in the exercise condition showed a significant improvement on both tasks, with a better outcome for the choice compared to the simple task. These findings indicate that physical exercise also has an impact on cognitive functioning in children.