A large body of research indicates that child development is sensitive to early‐life environments, so that poor children are at higher risk for poor cognitive and behavioral outcomes. These developmental outcomes are important determinants of success in adulthood. Yet, remarkably little is known about whether poverty‐alleviation programs improve children’s developmental outcomes. We examine how a government‐run cash transfer program for poor mothers in rural Ecuador influenced the development of young children. Random assignment at the parish level is used to identify program effects. Our data include a set of measures of cognitive ability that are not typically included in experimental or quasi‐experimental studies of the impact of cash transfers on child well‐being, as well as a set of physical health measures that may be related to developmental outcomes. The cash transfer program had positive, although modest, effects on the physical, cognitive, and socioemotional development of the poorest children in our sample.