A number of years ago, I was in a bookstore perusing the chapter book section to see what was popular at that time. When a young girl presented a book from this series to her mother, the mom refused to buy it and told her daughter she had read "enough fairy books" and had to select something else. The child was disappointed and I thought the mother was being too strict, possibly even preventing her daughter from developing a lifelong love of reading. Now that I've read four books from this series myself, I've completely changed my mind - kudos to that mother for steering her child in a better direction. 
 The argument is that children want to read these books, and at least they are reading something. Normally I would agree with that - any reading is better than none at all. But these chapter books are such poor examples that I'm actually not convinced that reading this is better than not reading anything. At least Captain Underpants exemplifies characterization, contains humor, has a plot, and follows proper story structure. All of those elements are missing from here. 
 So what attracts children to the books in this series like a dog to bacon? In every one of the four books of this series that I read, the two insta-friends get fairy dust sprinkled over them, shrink to fairy size, and grow wings. Never mind that it only lasts for an instant, and they don't have any real adventures while impersonating fairies. The books in this series capitalize on every little girl's desire to be a fairy, every child's wish to be magical. It's crass commercialization at its best. With zero literary value.