Compelling evidence supports the foundational importance of early self-regulation (SR). It also supports parents in the home environment as having the foremost influence on early development. Yet, prevailing approaches to support early SR growth have tended to leverage early education and clinical settings. Partners in Play (PiP) was developed as a sustainable approach for parents to learn how and when to support children through experiences of self-regulation challenges in the home learning environment. This study reports the first randomised control trial evaluation of the PiP program, with 21 parent–child dyads (consisting of twelve girl–mother dyads, eight boy–mother dyads, and one boy–father dyad; mean child age = 4.12 years, SD = 0.65). Dyads were randomised to a PiP intervention group (n = 10), which entailed four online parent information sessions and four out-of-home guided practice dyadic play sessions across 8 weeks, or an active control group (n = 11). The primary outcome was parent autonomy support, and the secondary outcome was child SR. Results indicated a significant increase in parents’ use of autonomy support and a non-significant but moderate-sized effect on child SR. This innovative proof-of-concept program and evaluation provides a roadmap for effecting change in parental support during children’s play, to the prospective benefit of important abilities such as child SR. Analyses show promise for a parent-based model toward parent behaviour change and child SR improvement.