Loathe? I once heard 'hate' described as the opposite of 'love - love with its back turned, if you like. At any rate, hate seems to require some sort of active emotional investment. It's a hot sort of feeling.

Loathe, on the other hand, is dislike turned up to eleven, with elements of contempt. A cold sort of emotion.

To me, despise is a different sort of emotion altogether. It's just a complete lack of respect: you can despise someone without disliking them, and certainly without hating them. In fact, if you despise someone you probably don't respect them enough to hate them or loathe them.

On the other hand, two things occur:
1. If you are writing for children, you need to use words they understand, and will also get across the relationship between the Snarls and the Combs etc that you want to portray.
2. The book is yours. Before cutting out words from your vocabulary, make sure that it's really what you want to do. Parents have all sorts of opinions about what they will and won't allow their children to see, hear and otherwise experience - some of which opinions are not commonly shared. Some parents also don't believe in allowing their children to learn that meat comes from animals, for instance.

But then, I just read books; I don't write them... :-)