This clinical trial studies a mobile health fitness program for adolescent and young adult childhood cancer survivors. Adolescent and young adult childhood cancer survivors are at risk to have negative late effects from treatment and to develop chronic health conditions. A sedentary lifestyle may increase the risk of cardiovascular disease, osteoporosis, and early mortality. Physical activity reduces the risk for cardiovascular disease and early mortality, improves cardiorespiratory fitness, muscular fitness, bone health, and body composition, and it is also positively associated with quality of life. Programs and technologies that promote physical activity are important because health behaviors adopted by adolescent and young adult childhood cancer survivors are likely to continue into adulthood. A mobile health fitness application may motivate adolescent and young adult childhood cancer survivors to engage and maintain physical activity.