Symbolism, Imagery, Allegory
There is a heck of a lot of eating going on in The Witches. If you think about it, some of the main events seem to center around food. The witches plan to deliver their Mouse-Maker through chocolates and other sweets, and, in the end, they get Mouse-Made themselves by eating their soup at dinner. Think of some of the other food-related moments in the story, too: there's Bruno, the oaf, who's always eating, and the cooks who spit in the meat of a customer... yuck.
Basically, all of our associations with food in this book are negative. That's surprising, coming from the guy who wrote Charlie and the Chocolate Factory, but the message here is no different than the message there: don't be greedy. Food and sweets in moderation are wonderful things, but go overboard and you're in a world of trouble. The witches know that children will come running when they hear there are free sweets to be had, and, when they do, they'll be turned into mice. Bruno's overeating is what gets him turned into a mouse in the first place, too – remember, he comes to the witches' meeting for the promise of more chocolate. Perhaps we're supposed to learn from all of this that food should be appreciated, not abused.