Purpose: Morphological regularities are an important feature of the English writing system, and exposure to written morphology may be key in the development of skilled word recognition. Our aim was to investigate children's experiences of written morphology by analyzing a large-scale corpus of children's reading materials spanning a target age range from 5 to 14 years. Method: Analysis was based on the Oxford Children's Reading Corpus. We examined frequency distributions of derived and compound words by target age and genre, as well as type and token frequencies for individual derivational suffixes. Results: We found that the proportion of morphologically complex words -- and derived words particularly -- increased in line with target age, and that nonfiction contained more complex words than fiction. Frequencies of individual suffixes also varied by target age and genre, with Germanic forms more common in fiction and texts for younger children, and Latinate forms more common in nonfiction and texts for older children. Conclusion: These findings provide a comprehensive picture of how children's experience with written morphology changes over the course of reading development. We discuss these findings in the context of developmental changes in morphological processing, and the benefits and limitations of using large-scale natural language datasets.