Most children with cancer are treated with chemotherapy. Eight in every ten of these children develop a sore mouth, which can involve extensive, painful ulceration. We call this oral mucositis. When receiving chemotherapy, a special red light can be shone into the mouth, which reduces the risk of painful ulcers by about half. We call this photobiomodulation, or PBM for short. PBM is recommended by many national and international guidelines, but in the paediatric guideline there is uncertainty around how acceptable this treatment is for children and young people. This study involves interviewing children and young people, their parents, and healthcare professionals to find out their experiences of oral mucositis and of PBM. It aims to determine the acceptability of PBM to children and families, to address uncertainty in this area.