Children begin interacting less across racial lines around middle childhood, but it remains unclear why. We examine the novel possibility that, at that time, children's prejudice theories—their understanding of prejudice as a fixed or malleable attribute—begin to influence their desire for interracial affiliation. We devise immersive behavioral experiences to evaluate when and how prejudice theories affect interracial affiliation. Study 1 measured prejudice theories among 8–13-year-olds (N = 152; 76 White, 76 racial minority) and observed children in a newly-developed social interaction task. In line with our hypothesis, children older than 10 years with stronger malleable-prejudice theories exhibited more interest and affiliation in a simulated cross- (vs. same-race) interaction, regardless of their preexisting prejudice level. Study 2 randomly assigned children to listen to a fixed- or malleable-prejudice theory story before engaging in a real, first-time interaction with a same- or cross-race partner at a different school via live video-stream (N = 150; 96 White, 54 racial minority). The malleable theory increased children's interest in further interaction with their cross-race partner. These findings highlight the promise of malleable-prejudice theories for sustaining positive interracial relationships during a critical developmental window—when the frequency of cross-race friendships typically declines.