It's not too hard to figure out why The Witches qualifies as children's literature: it's written for children. In fact, the narrator even points this out. Right at the beginning, he tells the reader some ways to recognize a witch so that they can avoid being caught by a one while they're still young (1.29). Just because it's children's literature, though, it doesn't mean it's not fun for adults. Shmoop is an adult and The Witches is one of our absolute favorites.
Adventure and fantasy sometimes go hand in hand. We all know what adventure is: something exciting (check) where there's usually a risk involved (check). This is also a fantasy, though, because it takes place in a world that's not quite like our own. We know it's not a fairy tale – that is made very clear to us at the beginning of the book: ahem, "this is not a fairy-tale." (1.2). Still, there are a few things in the world of The Witches that we just have to accept as reality, like, say, grobblesquirts – or, you know, witches.