Despite the potential benefits of children's confrontations of other children's racial biases--especially for targets of bias--little is known about how young children react upon observing instances of racial discrimination. In the present research, child participants completed a novel measure designed to test their reactions to another child's racially discriminatory behavior. The measure presented scenarios in which a protagonist who matched the participant's race (Asian, Latinx, or White) repeatedly excluded Black children from different social activities. Participants evaluated the protagonist's behavior and had an opportunity to confront the protagonist. Both a pilot study and a full preregistered study revealed that the novel measure had high reliability within participants and substantial variability across participants (pilot study: N = 54 U.S. White 5-7-year-olds, 27 girls, 27 boys, median household income range of $125,001-$150,000; full study: N = 126 U.S. 4-10-year-olds, 33.33% Asian, 33.33% Latinx, 33.33% White, 56 girls, 70 boys, median household income: $120,001-$125,000). In the full study, older children and children whose parents reported more racial socialization rated the protagonist's behavior more negatively; older children were also more likely to confront the protagonist. Neither participants' own race nor their prior exposure to racial diversity impacted their evaluations or confrontations of discrimination. The results have implications for understanding children's potential to serve as agents of social change by regulating other children's racial biases and behaviors.