Once you've become familiar with some of the research in this area, you'll need to start looking for the gaps in the research.
When searching for "Physical therapy and childhood cancers", I see that there are a few articles about quality of life, play based therapy, exercise intolerance, etc. Some research questions for this topic might be:
How can play based exercise improve the quality of life in leukemia patients?
Do survivors of childhood cancer struggle to be physically active?
Does radiation therapy or chemotherapy make it more difficult for cancer patients to remain physically active?
Does physical therapy help improve outcomes in cancer patients undergoing radiation therapy or chemotherapy?