Young children’s mathematics development underpins cognitive development, building brain architecture, fostering problem-solving, puzzling, and persevering, and strongly impacting and predicting future success in school – but systemic opportunity gaps have created unequal access to high-quality mathematics learning experiences, with differences in children’s math knowledge beginning before kindergarten entry. The Young Mathematicians project has partnered with families, teachers, and early childhood programs in richly diverse communities with large populations of students of color, linguistically minoritized students, and students living in poverty, to research and develop a playful developmentally-appropriate intervention that supports math learning across home and school environments.