This study examined the longitudinal development of young children's informal understanding of division through fair sharing. Two hundred and twelve children were followed from the age of 4.5–5.5 over three waves of assessment. Division understanding was measured with different types of problems (same- versus different-divisor trials) presented with different situations (partitive vs quotitve). Several key findings emerged. First, different-divisor problems were significantly more difficult than same-divisor problems at all age levels, which suggests that tasks that demand children's understanding of the inverse relation between divisor and quotient are more difficult than tasks that demand the understanding of equivalence principle. Second, children found it easier to understand the inverse relation between divisor and quotient in the context of partitive than quotitive situation. Third, with age, there was a sharper improvement in solving different-divisor problems than same-divisor problems, whereas the progress was more prominent for different-divisor problems presented in partitive situations, compared with other problems. Finally, working memory made the most consistent and unique contribution to solving all kinds of division problems at all age levels, compared with other cognitive factors. Theoretical and practical implications in early childhood mathematics education are discussed.