The current study investigated the development of numerical estimation in 3- to 5-year-old children sampled monthly for six months. At each session, children completed a task that assesses verbal number knowledge (Give-N task) and a numerical estimation task that assesses approximate number knowledge (Fast Cards). Results showed that children who acquired the cardinal principle (CP) during the course of the study showed marked improvement on the estimation task. Following CP acquisition, estimation became more accurate overall but also fluctuated widely. We discuss the implications of our findings for number word learning, particularly the mapping between verbal number and the approximate number system (ANS).