There was a need to develop a mucositis instrument that would be specific for use with children. This paper describes the step of generating items in the process of developing a new instrument for the assessment of oral mucositis in children. Nine health care professionals with expertise in pediatric cancer, mucositis assessment, and oral assessment in children were invited to participate in a nominal group technique to generate items that should be included in an instrument. Thirty items were generated initially. Voting processes established that six of these items were thought to be necessary for inclusion: (1) presence of ulcers, (2) pain assessment, (3) amount of pain medication received, (4) effect on eating, (5) drooling–pooling of saliva, and (6) effect on drinking. Using these six items, an initial draft of an instrument was developed that could be used to assess oral mucositis in children, namely, the Children’s International Mucositis Evaluation Scale. The six items generated by this process provided the basis for a simple, feasible, and reliable instrument. With increased clinical research investigating interventions to reduce and prevent oral mucositis, such an instrument will be critical to the effective conduct of this research in children. Further testing of this instrument is necessary.