A large literature suggests that the organization of words in semantic memory, reflecting meaningful relations among words and the concepts to which they refer, supports many cognitive processes, including memory encoding and retrieval, word learning, and inferential reasoning. The co-activation of related items has been proposed as a mechanism by which semantic knowledge influences cognition, and contemporary accounts of semantic knowledge propose that this co-activation is graded--that it depends on how strongly related the items are in semantic memory. Prior research with adults yielded evidence supporting this prediction; however, there is currently no evidence of graded co-activation early in development. This study provides the first evidence that in children the co-activation of related items depends on their relational strength in semantic memory. Participants (N = 84, age range: 3-9 years) were asked to identify a target (e.g., "bone") amid distractors. Children's responses were slowed down by the presence of a related distractor (e.g., "puppy") relative to unrelated distractors (e.g., "flower")--suggesting that children co-activated related items upon hearing the name of the target. Importantly, the degree of this co-activation was predicted by the strength of the target-distractor relation, such that distractors more strongly related to the targets slowed down children to a larger extent. These findings have important implications for understanding how organized semantic knowledge affects other cognitive processes across development.