Recent work suggests that common knowledge is an important cognitive mechanism for coordinating prosocial behavior, in part because it reduces uncertainty about others' cooperative behavior. However, it remains unclear whether children also rely on common knowledge to solve coordination problems. Here we examined whether 6- to 9-year-old children (N = 133) from the United States were more likely to attempt to coordinate when they had common knowledge about a joint payoff. Participants saw 3 vignettes that modeled the structure of a 2-player coordination problem and were provided with common knowledge, secondary knowledge, or private knowledge about the mutually beneficial, but risky, joint payoff. By 6 years of age, participants were more likely to attempt to coordinate when they had common knowledge than secondary knowledge, and secondary knowledge than private knowledge. Participants were also most likely to expect the other player to coordinate, and were most certain in their predictions, when there was common knowledge. Results indicate that, by middle childhood, children are able to solve coordination problems by relying on common knowledge, in part because it likely increases their certainty in others' cooperative behavior. Overall, findings suggest that common knowledge is an important cognitive mechanism for coordinating behavior and that it does so by reducing uncertainty about others' cooperative behavior.