Language documentation efforts are most often concerned with the adult language and usually do not include the language used by and with children. Essential parts of the natural linguistic behaviour of communities thus remain undocumented, and a growing body of literature explores what language documentation, language maintenance, and language revitalization have to gain by including child language and child-directed language. This paper adds a methodological perspective to the discussion, arguing that child language and child-directed language constitute data types that can inform our understanding of the adult language. For reasons of feasibility, the paper focuses on child-directed language only. Presenting data from two on-going language acquisition projects (Qaqet from Papua New Guinea and Dëne Suline´from Canada), we illustrate how this data type provides insights into the metalinguistic knowledge of adult speakers. After an introduction to child-directed language, three case studies on the topics of variation sets, clarification processes, and discourse context are exemplified from both languages and related to our understanding of the adult language. Focusing on the potential of this data type, this paper argues in favour of extending our documentation efforts to events involving children.