We evaluate the adverse effect of growing family size on four measures of children’s cognitive and health outcomes using Australian data. We utilise semi-parametric tests of quantile treatment effects to evaluate potential heterogeneity in this ‘quantity-quality trade-off’ across the distribution of outcomes. Identification is achieved through two instruments that are heavily used in the literature. Our results demonstrate the existence of a trade-off among Australian children, which is robust across outcome measures, but stronger for cognitive outcomes. The impact is uniform and affects more or less all children irrespective of their position in the outcome distribution.