Children continuously engage in developing theories about how the world works as they inquire through play and interactions with others. In this study, we investigated children’s in-process thinking in the form of working theories about language as they participated in engagements around dual language picturebooks in an afterschool club. Our research is framed within sociocultural theories on knowledge building, inquiry acts and recent research on working theories, dual language picturebooks, and language awareness. Although dual language picturebooks are often associated with supporting bilingual learners in learning a language, we argue that these books can also encourage children in learning about language and language diversity. We analysed field notes, audio recordings, and children’s artefacts to identify the kinds of working theories about language explored by children and their strategies for developing these theories. The findings show children explored working theories about what language is, what it means to know a language, and how language is learned. The strategies children used to develop these theories included demonstrations of peers and adults, the mediation of tools, talk as a tool of mind, and language play. This study contributes to understandings about how children develop working theories about language and language diversity through inquiry and suggests possibilities for classroom practice in opening spaces for children to connect to what they consider significant in engagements with dual language picturebooks. (author abstract)