Sylvia Lasalandra, author of the book A Daughter's Touch, has recently published a children's book based on her experience with postpartum depression. It's called "Mommy, Were You Happy the Day I Was Born?" She created the book especially for her daughter Melina. One of the lovely things about it is that she doesn't go into the gory details of PPD, which I don't think is necessary to do with a young child, but very briefly touches on "not being able to show love the way she wanted to" when her daughter was an infant.
Here's a story written by Susan Stone about the book for the website EmpowHer, with information on how to order it if you are interested.