Objective Complementary and alternative medicine (CAM) is commonly used by children with juvenile idiopathic arthritis (JIA), yet no validated questionnaires assess that use. The objective of this study was to develop child self- and parent proxy-report questionnaires assessing CAM use and to determine the face and content validity of the “Which Health Approaches and Treatments are you using?” (WHAT) questionnaires in pediatric rheumatology. Methods A sequential phased mixed methods approach was used to develop the questionnaires. A Delphi Survey of 126 experts followed by an interdisciplinary consensus conference of 14 stakeholders in CAM, general pediatrics and pediatric rheumatology was held to develop consensus on the content of the questionnaires using a nominal group technique. To determine face and content validity of the questionnaires, two groups, including (a) a purposive sample of 22 children with JIA 8 to 18 years and their parents from the Children’s Hospital of Eastern Ontario and the Hospital for Sick Children, and (b) 21 Canadian pediatric rheumatology experts, participated in interviews. Participants were independently asked about the goal, understandability and comprehensiveness of the WHAT questionnaires, as well as the relevance of items. Results Consensus was reached on 17 items of the WHAT questionnaires. The domains found to be relevant were child’s CAM use, factors associated with CAM use, perceived impact of CAM use, and communication about CAM. A total of 15 items in the parent proxy-report questionnaire and 13 items in the child report questionnaire showed adequate content validity. Conclusions Consensus was reached by experts on the content of a pediatric CAM questionnaire. Face and content validity testing and modifications made to the WHAT questionnaires have helped ensure adequate preliminary validity for use in pediatric rheumatology. This constitutes the basis for further testing of these questionnaires in pediatric rheumatology and for adaptation to other chronic diseases.