Objective: This study aimed to test the inter-rater reliability, construct validity, and feasibility of the modified “Which Health Approaches and Treatments Are You Using?” (WHAT) questionnaires in pediatric oncology. Methods: A sample of parent-child dyads was invited to complete self- and proxy-report modified WHAT, Pediatric Quality of Life Inventory, demographics, a diary of child recent use of CHA, and a questionnaire assessing aspects of feasibility. Parents were also asked to complete a satisfaction of children’s use of CHA survey. Results: Twenty-four dyads completed the study. The mean weighted kappa showed strong inter-rater reliability (k=0.77, SE=0.056); and strong agreements between the modified WHAT and the diary (self-report [k=0.806, SE=0.046], proxy-report [k=0.894, SE=0.057]). Significant relationships were found only between recent and non-recent CHA users in relation to easy access to CHA (self-report [p=0.02], proxy-report [p<0.001]). The mean scores of the feasibility scale (out of 7.0) for the self- and proxy-report were 5.64 (SD=0.23) and 5.81 (SD=0.22), respectively, indicating the feasibility of the modified WHAT. Conclusions: The findings of this study provide initial evidence of the reliability and validity of the modified WHAT questionnaires and their feasibility in assessing child CHA use and initiating clinical discussions about CHA with children and their parents. Further research is needed to test the theoretical relationships and to further explore the validity and reliability of the modified WHAT.