Background: Children’s preferences for activities are one of the most important predictors for participation. Currently no web-based measure of activity preferences exists for children with disabilities. Aim: To develop and investigate feasibility and internal consistency of a new web-based measure of activity preferences, ActiveYou I for children with disabilities. Materials and methods: The ActiveYou I was developed in three steps using a mixed-methods design. A review of the preferred goal activities of 149 children with disabilities was used to identify relevant activities. A pilot version of ActiveYou I was tested with 341 children with disabilities. Semi-structured group interviews and cognitive interviews were conducted with therapists and children with disabilities. Results: Nineteen physical activities were included in the pilot version. The pilot trial resulted in two activities being excluded, and one activity added, leaving the instrument with eighteen activities. Internal consistency was acceptable (Cronbach’s α ≥ 0.77). Interviews with therapists and children showed that ActiveYou I included relevant activities and was easy to answer. Conclusion: ActiveYou I proves to be a valid and feasible, web-based instrument for mapping activity preferences in children and adolescents with disabilities. Significance: Preferences are an important mediator for participation; consequently it is essential to seek activities that match the children’s preferences.