When you really think about it, The Witches is a pretty stinkin' violent book. Our narrator is violently grabbed and held down while poisoned with Mouse-Maker. We see a witch get blown into smoke right in front of our eyes (or at least our narrators' eyes). Plus, the witches talk about countless violent ways in which they like to kill children. Here's the thing, though: it still seems to be lighthearted. This is the magic of Roald Dahl. How does he do it? See the sections on "Tone" and "Writing Style" for our thoughts. Unlike in other books, violence in The Witches doesn't make you cringe. Sometimes it even makes you chuckle because of how absurd it is. If you want to really have the point driven home, watch the movie version of the book, and you'll see how much more... violent... the violence is, because it's missing Dahl's lighthearted storytelling techniques.
Violence is violence is violence. Roald Dahl shouldn't have treated it so lightly.
The witches talk a bigger game than they play. They're not as violent in their actions as they are in their words.