This paper aims to advance our understanding of how children's use of vocabulary in writing changes as they progress through their school careers. It examines the extent to which a model of lexical sophistication as use of low-frequency, register-appropriate words adequately captures development in vocabulary use across the course of compulsory education in England. We find that the received model needs elaborating in a number of important ways. Specifically: (1) the average frequency of words in the repertoire used by older children is no lower than that of younger children. However, younger children's writing is characterized by extensive repetition of high frequency verbs and adjectives and of low frequency nouns (the latter being a product of a focus on entities which are rarely discussed in adult writing). The role of repetition in this finding implies that lexical sophistication is inseparable from lexical diversity, a construct which is usually treated as distinct. (2) Younger children's writing shows a preference for fiction-like vocabulary over academic-like vocabulary. As they mature, children come to make greater use of academic vocabulary in both their literary and non-literary writing, though this increase is greatest in their non-literary writing. Use of fiction vocabulary remains constant across year groups but decreases sharply in non-literary writing, showing an enhanced sense of register appropriateness. This development of register appropriate word use can be captured by relatively simple frequency-based measures that could readily be employed by teachers and researchers to track writers' development in this aspect of word use.