We examine the relationship between family size and children’s human capital, commonly known as the quantity–quality trade-off in Latin America. Despite historically high fertility rates and a rapid convergence toward levels observed in high-income countries, evidence on this trade-off in the region remains limited. Using census data from five countries and a twin instrumental variable strategy, we estimate the causal effect of family size on educational outcomes. We document a trade-off in households with three or more children: an additional sibling increases the likelihood of falling behind in school, reduces primary school completion, and lowers total years of schooling. The magnitude of the trade-off is related to tighter financial constraints, more traditional gender norms, and insufficient local school supply. Identifying the contexts in which these trade-offs arise is essential for understanding how fertility patterns shape gaps in human capital accumulation and for informing policies aimed at narrowing intergenerational disparities.