Sing & Grow is a 10-week group music therapy intervention to promote positive parenting and child development for marginalized parents of birth to 3-year-old children. This paper examined whether changes from pre to post intervention varied according to implementation site, when the intervention was taken to scale nationally. Outcomes for 850 participants were compared for the site where the program was first established against three new locations; one site where implementation processes were more favorable relative to the other two sites. Overall, the findings provided only limited support for differential outcomes by site of implementation. Participants showed significant improvements in parent-reported parenting and child outcomes from pre to post that were similar across all sites. For clinician-reported outcomes, improvements over time were generally greater in the original site and the well-supported site compared to the sites where there were more implementation difficulties. These differences were partly accounted for by differences in the characteristics of participants receiving programs in different sites and differences in the clinicians' ratings of program quality and the levels of support and training provided. However, confounding by the source of measurement requires cautious interpretation of clinician data. This study further highlights the potential for music therapy as an early parenting intervention, and the need for more rigorous evaluations in this field.