To codesign, develop, and evaluate a smartphone app that includes patient-reported measures of symptoms and real-time advice in children's cancer. The Oncology Hub is a comprehensive approach to symptom management that includes a suite of codesigned tools and resources including clinical algorithms to determine the level of concern, symptom management advice, and resources for families of children with cancer. The evaluation involved Think Aloud interviews with parent and adolescent patients to complete tasks in the app as well as a User Experience questionnaire (score range, 0-120) and qualitative feedback. The accuracy of algorithms was determined by repeated testing of inputs and outputs over 4 weeks. Design and wireframes were iteratively refined through consultation with parents and adolescents confirming the final design. Beta testing evaluation was then completed by 25 participants including two adolescents. Across all participants, 84% of tasks were easy to navigate, and the Oncology Hub demonstrated high usability, usefulness, and acceptability with participants' scores ranging between 90 and 120 (mean = 112.2, standard deviation = 9.43). Qualitative feedback was positive. Testing of algorithms identified inconsistencies in understanding between clinical research and coding teams; refinements were made until the expected response notifications were returned with 100% accuracy. Technology offers new ways to think about how clinicians and families communicate and share information to harness the best of community and hospital services. Understanding how information is exchanged using health apps, and how this affects clinical workflow is critical to successful implementation, and optimizing symptom assessment and management in children with cancer.