BACKGROUND: Children and young people are usually given liquid morphine by mouth for breakthrough pain, which can take thirty minutes to work. A faster-acting, quickly absorbed, needle-free pain medicine, that is easy to administer is needed such as transmucosal (sublingual, buccal, intranasal) diamorphine. Research evidence relating to the administration of medication for breakthrough pain in children and young people is limited. This study aims to describe the experiences and preferences of parents and/or children and young people regarding the route of administration of diamorphine, barriers and facilitators comparative to oral morphine, and participation in a randomised controlled trial. METHODS: In-depth, semi-structured interviews with parents and/or children and young people at home or hospital/hospice. RESULTS: Thirteen interviews with: nine mothers, one father, and three sets of parents jointly. No interviews took place with a child/young person. Most families had experience of the buccal route which was effective in ease of administration and time to control pain. The intranasal route was preferred by parents irrespective of experience. Parents' willingness for their child to take part in a trial depended on the time commitment, their child's pain trajectory and the stability of analgesic requirements. CONCLUSION: A randomised controlled trial of oral morphine versus transmucosal diamorphine would need to consider trial logistics, especially time commitment. Parents felt that the trial should be introduced initially by the clinical team, with written information from the research team, and sufficient time to ask questions. Patients who had discontinued oral morphine because of side effects, or those with gastrointestinal failure, should be excluded. Maintaining stability in pain management was essential to families, so the timing of the trial is a potential issue.