As they develop, children become interested in and use increasingly diverse apps on devices such as smartphones. For more than a decade, pediatricians, developmental psychologists, and computer scientists like us have been studying children's ability to use smartphone apps and the positive and negative effects of doing so. In this article, the authors identify five principles that govern the use of smartphones by children. Parents need to be aware of the positive and negative effects of children using smartphones, help children choose apps, and co-use the apps with them occasionally.