The present study examined children's co-construction of shared meanings and internal state language during play with their sibling and friend from early to middle childhood. Sixty-five 4-year-old children (T1) and 46 7-year-old children (T2) were observed in the home during two free play sessions with a sibling and with a friend at both time points. Shared meaning strategies during social play (e.g., extensions and positive responses) and during pretend enactment were coded. Shared meaning strategies were associated with relationship context; children used more positive shared meaning strategies with friends compared to siblings and more introductions with siblings than friends. Moreover, developmental differences were evident; specifically, children employed more simple strategies, semantic tying (e.g., building on to one's ideas) and prosocial strategies at T2 than T1. Differences were also evident during pretend enactment where children used simple strategies more at T2 than T1 and clarifications more at T1 than T2. Findings highlight children's sophisticated and flexible communication strategies used in the process of co-constructing a shared understanding during play.