Make her part of a story
During story time, ask her questions to keep his mind on task and focused on the characters: “What did the doggie do? What do you think he will do next? If you were in the story, what would you do instead?” Share your own thoughts and show him you’re enjoying the book, too.
If it’s not before bed, and you don’t mind revving up her energy level, have her play the part of one of the characters: crawling around on all-fours like the dog, or acting the part of the fairy princess. Physically acting out the character may help her stay on track longer than she could stay while sitting still.