In urban, industrialized cultures, the best predictor of how children acquire their native language is child-directed speech from adults. However, in many societies, children are much less exposed to such input. What has remained unexplored is the impact of another type of input: other children's speech. In cross-cultural head-turn experiments, we demonstrate that Shipibo-Konibo infants (Peruvian Amazon) and Swiss infants (urban industrialized setting) show greater attention to children talking among themselves than to adults doing the same. We further show that, despite hearing more child-directed speech than child speech, Swiss infants equally attend to child-directed speech by adults and child speech. Interestingly, child-directed speech and child speech share acoustic and structural features. These findings suggest that, if available, the speech of other children may play an important role in language acquisition.