This will be the first multidisciplinary, randomized, longitudinal trial of a tailored, parent- and child-focused physical activity program for children (ages 4- <19 years) with newly diagnosed ALL. It will test the ability of the intervention to prevent or diminish early physical function limitations and improve health-related quality of life (HRQL). The intervention will be tested for its effect on: 1) physical function outcomes (muscle strength, range of motion, endurance, gross motor skills), bone density and bone mineral content (end of therapy only); and 2) HRQL. This multi-site trial will test the intervention in 76 evaluable children with ALL (38 receiving the intervention and 38 receiving a placebo "minimal movement" standard care strategy).