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Both cognitive and developmental literatures report strong relationships between working memory and fluid intelligence. However, few authors integrate these data into a theoretical model of intelligence development in children. Relational complexity theory provides a theoretical framework: (a) to understand how working memory capacity could influence relational induction, which in turn constitutes a central mechanism of intelligence and (b) to create experimental tasks dedicated to the assessment of relational integration ability. In the present study, children aged nine to twelve were asked to carry out three tasks: a task assessing fluid intelligence, a classic working memory task, and a new task specifically designed to assess relational integration ability. Results indicate that age-related changes in fluid intelligence are mediated by the growth of relational integration ability, beyond the contribution of concurrent storage and processing. The discussion reexamines the respective roles of working memory functions in reasoning as well as methodological avenues enabling their study.