Children's hypothetical reasoning about a complex and dynamic causal system was investigated. Predominantly White, middle-class 5- to 7-year-old children from the Greater Toronto Area learned about novel food chains and were asked to consider the effects of removing one species on the others. In Study 1 (N = 72; 36 females, 36 males; 2018), 7-year-olds answered questions about both direct and indirect effects with a high degree of accuracy, whereas 5-year-olds performed at chance. Six-year-olds showed intermediate performance. Using food chains with clearer constraints, Study 2 (N = 72; 35 females, 37 males; 2020-2021) replicated these findings. These results indicate that the ability to think about hypothetical changes to dynamic causal systems develops between 5 and 7 years. Implications for science education are discussed.