So I went looking for a book at the library that she could understand, and I found No Nuts for Me by Aaron Zevy. It's a story about a day in the life of a boy with a nut allergy. The boy in the story is a bit older than Ella (he's elementary-aged), but she loved the book. The boy talks about what he can eat, how he uses his EpiPen, his MedicAlert bracelet, and lots of other stuff. By the time our 3-week library loan period was up, she had almost memorized the whole book!
I loved that the boy in the book focused on what he could do, rather than what he couldn't do. It shows kids that having a peanut allergy isn't the end of the world.
I would recommend the book for kids age 3 to 8 (although Ella understood it at 2).