Adults are capable of either differentiating or integrating similar events in memory based on which representations are optimal for a given situation. Yet how children represent related memories remains unknown. Here, children (7–10 years old) and adults formed memories for separate yet overlapping events. We then measured how successfully remembered events were represented and reinstated using fMRI. We found that children formed differentiated representations in the hippocampus—such that related events were stored as less similar to one another compared with unrelated events. Conversely, adults formed integrated representations, wherein related events were stored as more similar, including in medial prefrontal cortex. Furthermore, hippocampal differentiation among children and medial prefrontal cortex integration among adults tracked neocortical reinstatement of the specific features associated with the individual events. Together, these findings reveal that the same memory behaviors are supported by different underlying representations across development. Specifically, whereas differentiation underlies memory organization and retrieval in childhood, integration exhibits a protracted developmental trajectory.