Purpose: The aim was to culturally validate a questionnaire about children's/youth's participation to be used in a Swedish context. Methods: FUNDES-Child, based on the well-established CASP, was chosen. Questions about engagement and hindering factors were added to the existing questions about frequency and independence in 20 activity areas. Using a qualitative, explorative design, 16 interviews with children/youths/caregivers were made to explore opinions about the questionnaire. Follow-up interviews confirmed the result of the revised questionnaire. Qualitative content analysis was performed. Results: The interviews provided support for the questionnaire's relevance by being a tool to assess important aspects of participation, to gain insights into one's own/the child's participation, and to promote ideas about what causes the degree of participation. To achieve comprehensiveness, no activity area was found to be missing nor superfluous. However, some examples were needed to be modified where "parades" are unusual in Sweden and therefore removed, while "singing in choir" was added. In search for comprehensibility, opinions about the layout of the first version were raised and a varying degree of understanding of wording and concepts were found and thus taken into account. Conclusions: The questionnaire can be used for establishing meaningful goals and to potentially increase children's participation.