Testing the tradeoff between child quantity and quality within a family is complicated by the endogeneity of family size. Using data from the Chinese Population Census, this paper examines the effect of family size on child educational attainment in China. We find a negative correlation between family size and child educational attainment, even after we control for the birth order effect. We then instrument family size by the exogenous variation that is induced by a twin birth, and find a causal link between family size and children’s education. We also find that the effect of family size is more evident in rural China, where the public education system is poor. These findings suggest a quantity-quality tradeoff of children in developing countries.