Many models of learning rely on accessing internal knowledge states. Yet, although infants and young children are recognized to be proficient learners, the ability to act on metacognitive information is not thought to develop until early school years. In the experiments reported here, 3.5-year-olds demonstrated memory-monitoring skills by responding on a non-verbal task originally developed for non-human animals, in which they had to access their knowledge states. Children learned a set of paired associates, and were given the option to skip uncertain trials on a recognition memory test. Accuracy for accepted items was significantly higher than for skipped on a subsequent memory task that included all items. Additionally, children whose memory-monitoring assessments more closely matched actual memory performance showed superior overall learning, suggesting a correlation between memory-monitoring and memory itself. The results suggest that children may have implicit access to internal knowledge states at very young ages, providing an explanation for how they are able to guide learning, even as infants.