Professional development (PD) focused on social, emotional, and behavioral teaching practices has been identified as a critical need for preschool teachers. The Pyramid Model for Promoting Social Emotional Competence is a multi-tiered framework of evidence-informed teaching practices. Previous studies have shown that PD, which includes workshops, implementation supports, and controlled doses of Practice-Based Coaching, is associated with teachers’ improved fidelity of implementation of Pyramid Model practices. Some studies have shown corollary improvements in child outcomes. The present study reports corollary child outcomes from a cluster randomized efficacy trial examining effects of a PD intervention versus business-as-usual (BAU) PD. Ninety-two teachers and 997 children in these teachers’ classrooms, including 250 focal children at elevated risk for poor social, emotional, or behavioral outcomes, participated. Data on social, emotional, behavioral, and other developmental outcomes were collected pre-intervention and post-intervention for all children via teacher-completed rating scales. Data for focal children were collected on social interactions and challenging behavior using an observational coding system and for other developmental outcomes through direct child assessment. Results showed statistically significant and noteworthy effects for children whose teachers were in the PD intervention versus BAU PD for both non-focal and focal children’s social skills and challenging behavior. By post-intervention, focal children whose teachers were in the PD intervention condition showed greater rates of social interaction than their peers in BAU teachers’ classrooms. No statistically significant or noteworthy differences were found for other child development outcomes. Mediation of treatment effects based on teachers’ Pyramid Model practice implementation was evidenced for social and behavioral child outcomes. Moderator analyses for select teacher and classroom variables showed differential treatment effects for non-focal children’s challenging behavior in classrooms with higher child-to-adult ratios. Implications for future research and practice are discussed. (Author abstract)