English language teachers who work with children in schools need knowledge about how English works to construct and present knowledge. This article discusses three aspects of that knowledge: understanding variation in the registers needed to engage in different classroom tasks, being able to identify language features used in the disciplinary discourses of different school subjects, and being able to draw on meaningful metalanguage to raise children's consciousness about the ways English is used in the texts they read and write. Examples of these knowledge bases and how they are relevant in elementary and secondary classrooms are presented here, drawing on the meaning-based theory of systemic functional linguistics (SFL). Implications of the need to develop such understandings for the work of teacher educators are acknowledged. However, achieving this goal is argued to be especially important in school contexts where English language teachers are increasingly being asked to collaborate with and support subject-area teachers as they work with children learning English.