This study explored children's causal reasoning about the past and future. U.S. adults (n = 60) and 3-to-6-year-olds (n = 228) from an urban, middle-class population (49% female; [approximately] 45% white) participated between 2017 and 2019. Participants were told three-step causal stories and asked about the effects of a change to the second event. Given direct interventions on the second event, children of all ages judged that the past event still occurred, suggesting even preschoolers understand time is irreversible. However, children reasoned differently when told that the second event did not occur, with no specific cause. In this case, 6-year-olds and adults inferred that the past event also did not occur. In both conditions, inferences that future events would change emerged gradually between 4 and 6.