In this study, we examine preschool storybook apps to identify the types of characters represented in these stories and how they relate to design quality and opportunities for children to build their vocabularies -- a significant predictor of later success in learning to read. Specifically, we sought to understand (a) how storybook apps portray gender, race, and disability status; (b) how these characteristics compare in apps that contain the most and least opportunities to learn vocabulary; and (c) how these characteristics compare in apps with design features that might provide additional support for children's word learning. To do so, we conducted a content analysis of 70 best-selling preschool storybook apps in the United States. Results indicate that most protagonists in these apps are animals, while human characters are overwhelmingly white, able-bodied, and male, particularly in apps with the highest percentage of advanced vocabulary and design qualities that are most likely to support children's word learning. Thus, apps with the greatest learning potential lack human diversity, raising significant concerns for inclusive education.