It's my two-year-old's favorite book. If she could tell me why, my guess would be the repetition and toilet-filled fun speaks to her. I can't tell you how many times I've been asked, "Pee pee book?"
This seems like another good one in the series. I can't find a flaw or omission, though maybe someone else will. The tone (as in all the others) is cheerful, and the book makes toilet time seem fun!