As a core feature of musical rituals around the world, humans synchronize their movements to the pulse of a shared acoustic pattern—a behavior called rhythmic entrainment. The purpose of the present study was (a) to examine the development of rhythmic entrainment with a focus on the role of experience and (b) to follow one line of evidence concerning its adaptive function. We hypothesized (a) that children learn how to synchronize movements to sound during social interactions, where they experience this behavior as a convention of the surrounding culture’s practice, and (b) that rhythmic entrainment has an adaptive value by allowing several people to coordinate their actions, thereby creating group cohesion and ultimately promoting cooperativeness. We compared the spontaneous synchronization behavior of Brazilian and German preschool children during joint drumming with an experimenter, either vis-à-vis or separated by a curtain, versus drumming along a playback beat. Afterward, we measured the children’s prosocial tendencies toward the experimenter. We found that Brazilian children were more likely than German children to spontaneously synchronize their drumming in a social setting, even if the codrummer was hidden from view. According to hypothesis, the variation in individual synchronization accuracy between and within the two samples could be partly explained by differences in individual experience with active musical practice, as revealed by parental interviews. However, we found no differences in children’s prosocial tendencies depending on whether they just had drummed alone or together with the experimenter.