Across two experiments (N = 256), we test children’s ability to recognize similar causal structures among mechanical systems. In Experiment 1, 4- to 7-year-olds were shown unique sets of three machine types (a causal chain, a common effect, and a common cause) and asked to judge which machines were most similar. We find that 6- to 7-year-olds, but not 4 to 5-year-olds, spontaneously match machines that share the same causal structure. However, all children relied primarily on timing cues when making similarity judgments. In Experiment 2, we control for timing cues, instead asking children to discriminate causal structure by observing an intervention on each machine. We find that, in the absence of perceptual cues, only 8- and 9-year-olds successfully matched machines based on structural similarity. We discuss potential explanations for these findings and consider ways to support recognition of common causal structure in the learning environment.