The present study investigated children's internal state language during play with their sibling and friend across early and middle childhood. Specifically, the category type of internal state language (e.g., cognitions and goals), referent (e.g., own and other), and associations with children's birth order were examined. A total of 65 (T1: Time 1) and 46 (T2: Time 2) children were observed at age 4 and 3 years later when children were aged 7 with their sibling and with their friend in two separate play sessions at home. Videos were transcribed and coded for the frequency and category of internal state language. Children referenced cognitions more often with their sibling than friend at T2. Children's references to shared internal states (e.g., “we”) were more frequent at T2 than T1 across relationship contexts. Time effects were found when the category and referent of internal state language were cross‐tabulated (e.g., references to shared goals plus one's own cognitions were more characteristic at T2 than T1). Birth‐order effects were evident at T1 with younger children referencing goals and cognitions with their sibling more often than older children. Results provide new insight into children's use of internal state language across relationship contexts and developmental periods. Highlight The purpose of the present study was to compare children's category type and referent of internal state language across sibling and friend relationships and from early to middle childhood. Results demonstrated that children showed higher rates of cognitive terms at middle childhood, particularly with their sibling, and referred to shared internal states more at middle childhood than early childhood. Findings highlight the importance of considering social context and a longitudinal design when observing children's internal state language during play.