Children's increasing expertise in composition relies partly on word choice. Little is known about how children consider words as they write, their meta-lexical awareness, or about their choice of words for writing. In this study, we investigate children's meta-lexical awareness, as one aspect of their metalinguistic awareness, which guides their word choices as they write. We describe how child writers express their meta-lexical awareness and how this might relate to writing achievement. We view language as functional in context of use following systemic, functional grammar theorists. We employ a mixed methods approach using writing achievement scores and a modified think aloud protocol to understand what children think about words as they write. Findings show that children develop increasing sophistication in the way they consider and choose words and that this relates to their writing achievement. The nature of children's meta-lexical awareness and its relationship to writing achievement is theorised, adding to the growing body of research into the importance of metalinguistic awareness to children's writing.