Adults and children show ingroup favoritism in their 3rd-party punishment of cooperative norm violations, suggesting that group loyalty importantly shapes enforcement of cooperation. Ingroup favoritism additionally influences punishment of unfairness in the 2-party ultimatum game, in which people are directly affected by unfair behavior. However, the directionality of this relationship is unclear: In some cases, people are more forgiving of ingroup unfairness, whereas in others they are less forgiving. Here we aim to disambiguate this relationship by studying its origins in development, asking whether ingroup favoritism influences children’s offers to others and whether it affects their responses to being treated unfairly. Six- to 10-year-olds played a group-based ultimatum game after being assigned to minimal groups and made proposals to—and responded to offers from—members of their in- and outgroups. We tested children’s real bargaining behavior in the absence of deception. Results showed that, regardless of group membership, children’s primary concern lay with fairness: Participants regularly offered equal splits and were more likely to reject unfair offers than fair offers. Consistent with past work, older children made more generous proposals than did younger children. Although our group manipulation successfully induced ingroup bias in participants, neither children’s proposals nor responses were influenced by group membership. This suggests that second-party punishment of fairness norm violations is unbiased early in development and points to the potentially important role of experience with different groups in shaping later emerging bias in norm enforcement. We discuss implications for theories regarding when and to what extent group bias influences cooperation.