There’s recent evidence that children get leaner and fitter during the school term, but become fatter and less fit over the summer holidays. This may be because of differences in activity and diet: on holidays, children may eat more, be less active and have more screen time. We propose to track the time use, diet, fitness and fatness of 9-11 year old children across three school years. If the pattern is confirmed, we should focus more on the home environment, or perhaps encourage summer camps.