RecruitingBehavioural interventionPhysical Activity and Exercise During Early Treatment Phases for Childhood Acute Lymphoblastic Leukaemia to Protect Against Muscle Loss and Improve Frailty OutcomesThis is a small trial testing out a new approach before doing a bigger study. Researchers are observing a group of children/adolescents (ages 5-17) with acute lymphoblastic leukemia (ALL) and testing a physical activity and exercise program on a group of them who after 5 weeks of treatment show signs of weakness or frailty. Kids who are NOT losing muscle aren't part of the exercise trial - they're just monitored over time to see how they do. The goal: To see if an exercise program helps kids who are getting weaker from acute lymphoblastic leukemia treatment build back/maintain their strength, compared to kids who don't do the extra intervention. The study will also look at if this way of measuring muscle weakness works well for kids with cancer.