Background: Moderate-to-vigorous physical activity (MVPA) offers many health benefits for preschoolers (2.5–5 y). In childcare, MVPA is predominantly accumulated outdoors, with higher rates purported among children within the first few minutes outside. The Supporting Physical Activity in the Childcare Environment intervention included shorter, more frequent outdoor play sessions; this study sought to explore children’s activity levels during various outdoor play schedules. Methods: During the final week of the Supporting Physical Activity in the Childcare Environment intervention, preschoolers wore an Actical™ accelerometer for 5 days during childcare and staff logged outdoor times. Separate linear mixed effects models were run to explore the effect of the intervention on preschoolers’ physical activity (total and MVPA) and sedentary time during outdoor play. Sex was entered as an interaction effect. Results: Preschoolers (n = 292) were significantly more active in the first 10 minutes outdoors compared with remaining time (P < .0083). For total outdoor time, children in the experimental group engaged in significantly less sedentary time than those in the control group (P < .017), and experimental group boys and girls engaged in higher MVPA than boys and girls in the control group (P < .017). Conclusions: Findings support scheduling more frequent outdoor play sessions in childcare to increase physical activity participation among young children.