A review of 28 studies found that children aged 3-12 years who spend more time outside are more active and less sedentary than children who spend more time inside. This was apparent across ages, sexes, and contexts.
Gray, Casey, Rebecca Gibbons, Richard Larouche, Ellen Beate Hansen Sandseter, Adam Bienenstock, Mariana Brussoni, Guylaine Chabot et al. (2015). What Is the Relationship between Outdoor Time and Physical Activity, Sedentary Behaviour, and Physical Fitness in Children? A Systematic Review. International Journal of Environmental Research and Public Health, 12(6), 6455-6474.