Secrets play a powerful role in human social relationships. Here, we examine the developmental trajectory of 3- to 10-year-old children's (N = 630) expectations about (a) how relationships impact whether people will keep secrets, and (b) how relationships are impacted when a confidee keeps versus tells a confider's secret. Sophisticated expectations about the role of secrets in relationship maintenance develop across childhood. In particular, school-age children (6- to 10-year-olds) expect friends to be more likely to keep each other's secrets than nonfriends (Study 1), and expect that if a friend breaks this norm and shares his friend's secret with a third-party, it will harm the friendship (Studies 2 and 3). These expectations were specific to inferences about secrets: school-age children did not expect that sharing (or keeping) a friend's fact or surprise would impact the friendship strength (Studies 2 and 3). These findings did not hold for preschoolers (3- to 5-year-olds), who did not have clear expectations linking secret sharing to friendship strength. Taken together, our results indicate that by 6 years of age, children understand that social relationships can increase people's obligations to keep each other's secrets, and that failing to do so can harm the relationship.