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.