Language is a collaborative act: to successfully communicate, speakers must generate semantically valid utterances that are sensitive to the knowledge state of the listener. We asked whether parents’ spatial descriptions are tuned to their children’s spatial knowledge. Parent-child pairs (n=16, m child age 4;1) viewed identical complex spatial arrays on separate computer screens. Parents were asked to describe target objects so that their child could identify them on their own screen. Children’s knowledge of left/right was independently tested using a comprehension task. A hierarchical statistical model of the experimentally elicited spatial language predicted that the probability of parents using left/right was greater for children that achieved higher comprehension scores, indicating successful communicative adaptation. This result did not hold for parents of children with severe spatial impairments (Williams Syndrome), suggesting that there is considerable variation in how well parents tune their language to their children’s level of spatial language and knowledge.