We develop a positive theory of the adoption of child labor laws. Workers who compete with children in the labor market support a child labor ban, unless their own working children provide a large fraction of family income. Fertility decisions lock agents into specific political preferences, and multiple steady states can arise. The introduction of child labor laws can be triggered by skill-biased technological change, which induces parents to choose smaller families. The theory can account for the observation that, in Britain, regulations were first introduced after a period of rising wage inequality, and coincided with rapid fertility decline.