Hippocampal subregions and medial prefrontal cortex may differentially shape memory precision in the mature brain: While anterior and posterior hippocampus may store the general themes and specific details of an episode, respectively, medial prefrontal cortex may instead connect related memories. However, given continued change to the functionality of these regions beyond childhood, it is unclear how children’s memory precision is influenced by these same mechanisms. We characterized how hippocampal subregions and medial prefrontal cortex separately and in tandem encourage memory precision in childhood versus adulthood. Children (7-9 years old) and adults studied scene photographs and then performed a recognition test that included both the studied scenes and highly similar lures. Behaviourally, adults had more precise memories than children in that they were better able to discriminate studied scenes from lures. At the neural level, anterior hippocampus and medial prefrontal cortex were differently engaged during this memory formation: Anterior hippocampus engagement was related to subsequent memory across age groups, while children showed greater medial prefrontal cortex engagement than adults overall when studying scenes. Considering individual differences in engagement revealed further developmental differences. Children showed evidence for a trade-off in their reliance on posterior hippocampus versus medial prefrontal cortex during precise memory formation, suggesting competition between these regions. By contrast, the same structures in adults played a more cooperative role in supporting memory precision. These findings suggest that the relationship between posterior hippocampus and medial prefrontal cortex reverses over development to yield adult-like memory precision, such that these regions work in opposition in childhood before becoming specialized to cooperatively encourage precision in adulthood.