This paper examines the efficacy of a universally-offered parenting program, Tuning in to Toddlers (TOTS), that aims to improve parent emotion socialization, reduce parent and toddler stress and improve social, emotional, and behavioral functioning in toddlers. Three hundred parents of an 18–36 month old toddler were cluster randomized into intervention or control. Parents in the intervention participated in 6 × 2 h group sessions of TOTS. Baseline and 12-months post-intervention measures were collected using parent-report questionnaires and hair samples from parents and toddlers of systemic cortisol stress. Compared to controls, intervention parents reported significantly greater reductions in difficulties in emotion regulation (difficulty remaining goal directed: 95% CI.10, 1.71, p = .028; lack of access to strategies: 95% CI 0.62, 2.42, p = .001), emotion dismissing (beliefs: 95% CI 2.33,4.82, p < .001; behaviors: 95% CI 0.32, 0.65, p = <.001), greater increase in empathy (95% CI −2.83, −1.50, p < .001), emotion coaching (beliefs: 95% CI −2.56, −0.27, p = .016; behaviors: 95% CI −0.58, −0.24, p = <.001), children's behavior (95% CI 0.19, 2.43, p = .022) and competence (95% CI −1.46, −0.22, p = .008). Significant greater reductions in systemic cortisol were found for intervention but not control children (95% CI 0.01, 0.35, p = .041). Findings provide preliminary support for the use of TOTS as a universal prevention program to improve parent emotion socialization and children's functioning.Trial Registration: Australian and New Zealand Clinical Trials Registry: ACTRN12615000962538.