Heart problems are amongst the most common physical illnesses in children and young people (CYP). They can be present from birth or develop as CYP get older and are linked to increased physical and psychological difficulties overprotection from caregivers and healthcare providers and reduced quality of life. While adults are offered exercise classes and lifestyle advice after a heart problem, CYP with heart problems are not. Improving health behaviours in people with heart problems is vital, improves quality of life and reduces additional illnesses (i.e obesity, diabetes). Approximately 1 in 3 CYP with heart problems have anxiety and/or depression so it is also important to support their mental health. One way to do this is to develop and test the acceptability and feasibility of a trial of cardiac rehabilitation (CR) consisting of exercise with mental health support for CYP. The aim is to develop and test the feasibility and acceptability of a trial of a cardiac rehabilitation programme for CYP.