Aims: This study explores experiences of childhood cancer survivors and their parents with a combined physical and social activity intervention during treatment, including how the survivors and their parents perceive physical activity post-treatment. 

Design: A process evaluation using semi-structured interviews. 

Methods: Using a criterion-sampling strategy, 18 Danish childhood cancer survivors (aged 11–18 years) and their parents were interviewed from September 2019 through May 2020. Data analysis used an inductive thematic approach focused on meaning. 

Results: Three themes emerged: (1) being physically active during hospitalization; (2) peers as motivators and (3) physical activity post-treatment. During hospitalization, daily motivation to do physical activity was dependent on the daily well-being, that is, presence of the side effects from the child's treatment. Healthy classmates provided distraction, reduced loneliness and promoted normality for those hospitalized. For most of the survivors, their healthy peers provided motivation for being physically active during treatment. When surplus energy was lacking, some survivors preferred doing physical activity alone with a professional. Those who were physically active in the hospital sustained being physically active post-treatment while their parents continued seeking advice about appropriate activity levels. 

Conclusion: Childhood cancer survivors and their parents benefited from the intervention which also provided guidance to remaining physically active post-treatment. This was particularly true for the participants with leukaemia. 

Impact: Healthcare professionals should support children with cancer to be physically active during hospitalization. Including social and physical components in their care plan and being aware of individual preferences is pivotal to improving the survivors' level of physical and social well-being during and post-treatment. 

Patient or Public Contribution: The participants were involved in designing the interview guides to ensure that the interview guides were understandable for the participants to provide rich descriptions of their experiences with a physical and social activity intervention during hospitalization.