Third-party intervention (TPI) has been shown to emerge early in human ontogeny. However, little is known about how social relationship information influences children’s reasoning of TPI. The current study answered this question by exploring 6- to 11-year-old (N = 108) Chinese children’s reasoning of how a third-party observer would (descriptive norms) and should (prescriptive norms) intervene against unfair resource allocation, and how the reasoning was modulated by the peer relationships (friend, disliked peer, stranger) between the observer and unfair allocator. Results showed that peer relationships affected children’s expectations (would question) of TPI from age of 6, with this influence strengthening with age. However, children’s judgments (should question) of TPI were not affected by peer relationships or age. The results reveal that considerations of peer relationships drive children’s descriptive norms regarding TPI to increasingly diverge from prescriptive norms from age 6, deepening our understanding of the development of contextualized moral cognition.