The goal of this study is to learn if a parent-child dyadic physical activity (PA) intervention works to improve physical activity levels in children with leukaemia. It will also learn about the intervention's effects on children's self-confidence in doing physical activity and their quality of life. The main questions it aims to answer are:

Does the dyadic PA intervention increase the amount of physical activity children do, compared with an individual PA intervention or usual care? Does the dyadic PA intervention improve children's confidence in doing physical activity and their overall quality of life? What is the cost-effectiveness of the dyadic intervention relative to the individual intervention and psychological support control group?

Researchers will compare three groups: children and parents doing physical activity together (dyadic intervention), children doing physical activity individually (individual intervention), and children receiving psychological support sessions only (control group), to see if involving parents makes a difference.

Participants will:

Attend weekly sessions for 3 months (12 sessions total) Wear an activity tracker for 7 days and complete questionnaires at 5 different time points over 12 months Some participants will also be invited to take part in an interview about their experience