You should not be encouraging your children to read his books just because they are sold in supermarkets and are easily accessible. Would you encourage meanness in your children? That's what these books are. Same for Roald Dahl - they might be classics but they are not appropriate
My kids have grown up to be nice people who read for pleasure. I didn't restrict them to only reading children's books written by decent people with morally appropriate messages. They are articulate and write well, but I didn't restrict them to well-written books. I mostly let them choose what they read, as long as it wasn't inappropriate for their age. It's hard enough to get children reading and enjoying books, without banning certain authors for being nasty IRL, writing stories we don't like, or even for not writing well enough.
I do agree with pps saying that celebrity authors get far too much attention, and are a lazy way for publishers to sell books, but people generally aren't damaged by reading about reprehensible or stereotyped characters, and poor quality writing is still enjoyable for lots of children, in the same way a lot of adults would much rather read romances or thrillers than literary fiction.
Reading is a lifelong pleasure, as well as an important skill, so I can't understand why people want to put so many obstacles in the way of children doing it.