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We study how early child care (ECC) affects children's development in a marginal treatment effect framework that allows for rich forms of observed and unobserved effect heterogeneity. Exploiting a reform in Germany that induced school districts to expand ECC at different points in time, we find strong but diverging effects on children's motor and socio-emotional skills. Children who were most likely to attend ECC benefit in terms of their motor skill development. Children who were least likely to attend ECC gain in terms of their socio-emotional skill development, especially boys and children from disadvantaged families, such as those with low education or migration backgrounds. Simulating expansions of ECC, we find that a moderate expansion fosters motor skills for all children and language skills for boys and immigrant children. A progressive expansion of ECC improves all children's socio-emotional development but neither their motor skills nor their language skills.