This study tested the Wellness Enhancing Physical Activity in Young Children (WE PLAY) program, a 4-week online preschool teacher training, on children's moderate-to-vigorous physical activity (MVPA). In this cluster RCT, six Head Start preschools were randomized to an intervention and comparison group. Children's MVPA was measured using accelerometers at pre- and posttest. The magnitude of the difference in MVPA between groups at posttest was small, but in the expected direction: [delta] min/hour = 1.60, 95% CI [-0.97, 4.18], p = 0.22, Cohen's d = 0.32. We observed a pre/post within group increase in average minutes per hour of MVPA in school with a medium effect size for the intervention group: [delta] mean min/hour = 2.09, 95% CI [0.51, 3.67], p = 0.0096, Cohen's d = 0.42. An increase was not seen for the comparison group: [delta] mean min/hour = 0.44, 95% CI [-0.70, 1.59], p = 0.45, Cohen's d = 0.07. WE PLAY children in 6 hr/day programs gained 63 min of MVPA per week in school, providing preliminary evidence of the benefits of WE PLAY on children's physical activity levels. WE PLAY deserves further testing with larger groups of children and teachers.