Leisure-time physical activities (LTPA) can improve children’s physical fitness, their ability to control negative emotions, even their school performances. As the factors that influence schoolchildren’s willingness to participate in LTPA are multifaceted, an appropriate and valid measurement tool is critical for advancing the discussion of this topic. The purpose of this study is to develop a questionnaire about schoolchildren’s willingness to participate in LTPA and to verify its content validity and potential user validity. The research procedures involved three stages. The first stage was the development of the questionnaire. The questionnaire items were developed through systematic collection, review, and summary of the literature. The second stage involved eight experts in testing the content validity of the questionnaire. The third stage was to test the potential user validity with eight upper-grade students as questionnaire respondents. The systematic collection, review, and summary of the literature resulted in five factors: individual, interpersonal interactions, parents, environment, and school policies. Based on these, 36 items were drafted for the questionnaire, named “Influences on Schoolchildren’s Willingness to Participate in LTPA.” The questionnaire was validated by the eight experts. The item-content validity indexes (I-CVI) ranged between .88 and 1.0, and the summative content validity index (S-CVI) was .9. As for the items’ wording appropriateness, the I-CVI ranged between .75 and 1, while the S-CVI was .96. For potential user validity, the students rated the comprehensibility of the 36 items. Their responses fell between 3 (mostly understandable) and 4 (completely understandable). In conclusion, the expert content validity and potential user validity testing confirmed that the questionnaire has good content validity. In the future, the questionnaire will be administered to a large sample of schoolchildren to verify its construct validity and reliability.