It looks like a children's book. It has pictures. It has a saccharine-sweet title. The main characters are a little girl and her teddy bear. But all of that is just protective coloration. The truth is, this is a book for adults with a dark sense of humor and an appreciation of old-school faerie tales.
There are three separate endings to the book. Depending on where you stop, you are left with an entirely different story. One ending is sweet, another is horrible. The last one is the true ending, the one with teeth in it.
The Adventures of the Princess and Mr. Whiffle is a dark twist on the classic children's picture-book. I think of it as Calvin and Hobbes meets Coraline, with some Edward Gorey mixed in.
Simply said: This is not a book for children.