Background: State-level child care quality rating and improvement systems (QRIS) have been implemented in a majority of states in the U.S.A. One goal is to improve developmental outcomes for young children, especially from families with fewer resources. Research is needed to determine if QRIS can produce quality improvements that will support the development of children from birth to three years. Objective: Explore the association of toddlers' growth trajectories over two years with their participation in varying quality-rated levels of child care. Methods: In this two-year longitudinal study of 75 toddlers (M age = 15.5 months; SD = 2.36) children's cognitive and social-emotional growth trajectories were estimated in three assessments, 12 months apart. All toddlers were low-income Child Care and Development Fund voucher recipients, cared for by child care providers within a Midwestern state's voluntary QRIS. QRIS-rated child care quality, caregiver education level, and child--adult ratio were observed in parallel with individual cognitive and social-emotional child assessments. Growth analyses controlled for parent education, child race, ethnicity, disability status, and gender. Results: Toddlers who received care in settings rated highest by the QRIS and those with caregivers who had higher education levels progressed at significantly higher rates of development in early learning skills, compared with toddlers placed in QRIS-rated lower quality settings or with caregivers with less education. Conclusions: State-level QRIS have the potential to inform parents' and policymakers' decisions about child care quality associated developmental outcomes for children under three years of age. Especially important is the role of QRIS in incentivizing the educational preparation of toddler caregivers.