I don't think there is such a thing as a bad book for children. Every now and again it becomes fashionable among some adults to point at a subset of children's books, a genre, perhaps, or an author, and to declare them bad books, books that children should be stopped from reading. I've seen it happen over and over; Enid Blyton was declared a bad author, so was RL Stine, so were dozens of others. Comics have been decried as fostering illiteracy.
It's tosh. It's snobbery and it's foolishness. There are no bad authors for children, that children like and want to read and seek out, because every child is different. They can find the stories they need to, and they bring themselves to stories. A hackneyed, worn-out idea isn't hackneyed and worn out to them. This is the first time the child has encountered it. Do not discourage children from reading because you feel they are reading the wrong thing. Fiction you do not like is a route to other books you may prefer. And not everyone has the same taste as you.
Another gem from Neil Gaiman's lecture at The Reading Agency. I felt obliged to post this as a reminder to myself because I'm a horrible snob, especially about books.
My son currently loves books. Right now, the things he reads are all things I like. That's largely because Joanne and I make most of the book choices, but that's not going to last for long. He has more control than ever over what he reads, and rightly so. One day, he's going to choose something I don't like. And I'm going to have to keep my mouth shut.
The problem with being a snob is that you define yourself more by what you don't like than by what you do like. I'm going to need to get over my book snobbery quickly. A good starting point would be to simply ask people about the book they are reading. Rather the offering my opinion on the book when they haven't asked for it, I need to listen and repeat what they've told me. In doing so, perhaps I'll discover a different perspective on the book, rather than just repeating the same stale opinion that I've trotted out every single time the book comes up.