Oral morphine is frequently used for breakthrough pain but the oral route is not always available and absorption is slow. Transmucosal diamorphine is administered by buccal, sublingual or intranasal routes, and rapidly absorbed. To explore the perspectives of healthcare professionals in the UK caring for children with life-limiting conditions concerning the assessment and management of breakthrough pain; prescribing and administration of transmucosal diamorphine compared with oral morphine; and the feasibility of a comparative clinical trial. Three focus groups, analysed using a Framework approach. Doctors, nurses and pharmacists (n = 28), caring for children with life-limiting illnesses receiving palliative care, participated. Oral morphine is frequently used for breakthrough pain across all settings; with transmucosal diamorphine largely limited to use in hospices or given by community nurses, predominantly buccally. Perceived advantages of oral morphine included confidence in its use with no requirement for specific training; disadvantages included tolerability issues, slow onset, unpredictable response and unsuitability for patients with gastrointestinal failure. Perceived advantages of transmucosal diamorphine were quick onset and easy administration; barriers included lack of licensed preparations and prescribing guidance with fears over accountability of prescribers, and potential issues with availability, preparation and palatability. Factors potentially affecting recruitment to a trial were patient suitability and onerousness for families, trial design and logistics, staff time and clinician engagement. There were perceived advantages to transmucosal diamorphine, but there is a need for access to a safe preparation. A clinical trial would be feasible provided barriers were overcome.