The purposes of this study were to (a) examine the relationship between children's beliefs (value, competence, and goal orientations) pertaining to fitness-oriented activities and their participation in moderate-to-vigorous physical activity (MVPA), (b) determine whether children's own beliefs are related to their perceptions of their parents' beliefs about them (value, competence, goal orientations, expectancies), and (c) examine the relationship between children's perceptions of their parents' beliefs and their MVPA. Self-report questionnaires and structured interviews were administered to children (N = 81, ages 11–15) in a one-on-one situation in their homes to tap their own beliefs, their perceptions of their parents' beliefs, and to assess children's level of MVPA through the use of two-day recall and peer comparison measures. Multivariate multiple-regression analyses revealed that children's beliefs about their MVPA could predict a significant amount of the variance in their self-reported MVPA and that children's perceptions of their parents' beliefs were unrelated to their MVPA. Follow-up canonical correlation analyses showed that the most significant and positive predictors of MVPA were children's perceptions of competence and the degree to which they endorse a task and ego orientation. A second multivariate multiple-regression analysis revealed that children's perceptions of their parents' beliefs about them and their own beliefs about MVPA were significantly related. These findings are discussed within the framework of a Family Influence Model for understanding children's MVPA participation.