ABSTRACT: Early care and education settings, such as family child care homes (FCCHs), are important venues for children's health promotion. Keys to Healthy Family Child Care Homes evaluated a FCCH-based intervention's impact on children's diet and physical activity. This study enrolled 496 children aged 1.5-4 years and 166 FCCH providers into a cluster-randomized control trial (intervention = 242 children/83 FCCHs, control = 254 children/83 FCCHs) conducted during 2013-2016. The 9-month intervention addressed provider health, health of the FCCH environment, and business practices, and was delivered through three workshops, three home visits, and nine phone calls. The attention control arm received a business-focused intervention. Primary outcomes were children's diet quality (2 days of observed intakes summarized into Healthy Eating Index scores) and moderate to vigorous physical activity (3 days of accelerometry) at the FCCH. Secondary outcomes were child body mass index (BMI), FCCH provider health behaviors, and FCCH nutrition and physical activity environments and business practices. Repeated measures analysis, using an intent-to-treat approach, accounting for clustering of children within FCCHs and adjusting for child age, sex, and BMI, was used to evaluate change (completed in 2018). Compared to controls, intervention children significantly improved their diet quality (5.39, p = .0002, CI = 2.53, 8.26) but not MVPA (0.31, p = .195, CI = -0.16, 0.79). Intervention FCCH providers significantly improved their diet quality and several components of their FCCH environment (i.e., time provided for physical activity, use of supportive physical activity practices, and engagement in nutrition and physical activity education/professional development). FCCHs are malleable settings for health promotion, especially diet quality. TRIAL REGISTRATION: This study is registered at www.clinicaltrials.govNCT01814215.