Objectives. We sought to identify the program fidelity factors associated with successful implementation of the Buckle-Up Safely program, targeting correct use of age-appropriate child car restraints. Methods. In 2010, we conducted a cluster randomized controlled trial of 830 families with children attending preschools and long day care centers in South West Sydney, New South Wales, Australia. Families received the Buckle-Up Safely program in the intervention arm of the study (13 services). Independent observers assessed the type of restraint and whether it was used correctly. Results. This detailed process evaluation showed that the multifaceted program was implemented with high fidelity. Program protocols were adhered to and messaging was consistently delivered. Results from multilevel and logistic regression analyses show that age-appropriate restraint use was associated with attendance at a parent information session hosted at the center (adjusted odd ratio [AOR] = 3.66; 95% confidence interval [CI] = 1.61, 8.29) and adversely affected by the child being aged 2 to 3 years (AOR = 0.14; 95% CI = 0.07, 0.30) or being from a family with more than 2 children (AOR = 0.34; 95% CI = 0.17, 0.67). Conclusions. Findings highlight the importance of parents receiving hands-on education regarding the proper use of age-appropriate child restraints.