The primary aim of this study was to assess the efficacy of a childcare-based intervention in increasing child physical activity by allowing children unrestricted access to outdoor areas for free-play when structured activity is not taking place. A randomized controlled trial was conducted in six childcare services. Intervention services provided children unrestricted access outdoors for active free-play, while control services provided their usual scheduled periods of outdoor play. Consent was obtained from 231 children. Child moderate to vigorous activity (MVPA), the primary trial outcome, was assessed via accelerometer at baseline and 3 months post baseline. Intervention effects were examined using Generalised Linear Mixed Models. Controlling for child age, gender and baseline outcome measure, at follow-up there were no significant differences between groups in minutes of MVPA in-care (mean difference: 4.85; 95% CI: -3.96, 13.66; P = 0.28), proportion of wear time in-care spent in MVPA (mean difference: 1.52%; 95% CI: -0.50, 3.53; P = 0.14) or total physical activity in-care (mean difference in counts per minute: 23.18; 95% CI: -4.26, 50.61; P = 0.10), nor on measures of child cognition (P = 0.45-0.91). It was concluded that interventions addressing multiple aspects of the childcare and home environment might provide the greatest potential to improve child physical activity.