Despite the childhood benefits gained from engaging in adequate physical activity, Australian preschool-aged children are reported to spend little time being physically active. Parental planning is important to engaging preschool-aged children in physical activity. Behavioral barriers, normative support, and self-efficacy have been identified as key determinants of parental decision making. However, the interplay among these factors is not well understood. The aim of this study was to examine the role of these psycho-social variables on parents’ planning and behavior for their preschool-aged children’s physical activity. A survey at two points in time was conducted in 208 Australian parents (n  = 139 mothers, Mage = 36.43 years, SD = 5.04; n = 69 fathers, Mage = 36.33 years, SD = 6.5) of children aged between 2 and 5 years. A conditional process analysis was conducted that integrates mediation and moderation analyses. The results showed that planning and behavioral barriers predicted behavior, with a moderation effect also identified; more planning helped compensate for barriers. Normative support predicted both planning and behavior. In addition, an interaction between normative support and self-efficacy emerged. Parents with high self-efficacy engaged in high levels of planning, independent of normative support. Low self-efficacy with low normative support resulted in the lowest amount of planning; however, in the absence of normative support a high level of self-efficacy could compensate for it. Current findings provide avenues for future research to develop interventions to test the efficacy of these mechanisms in promoting parents’ ability to ensure preschool-aged children are active.