What is the best cat litter to use for kittens-in-training?

I have a mother cat and her five 2 week old kittens. The mother was a stray, but we took her inside our home. She is very sweet and she and her babies are now in a nice clean room. Anyway, in about one or two weeks the kittens will be learning to use the litter box. Right now I have just a regular clumping litter. But I’ve heard that certain litter types can be dangerous for kittens (and cats) if ingested, which is something those kittens will surely do. Should I use a clay litter? But sodium bentonite is a controversial subject…What about a natural pine or wheat litter? Should it be non-clumping? I’m going to ask a vet, but I thought I’d as everyone here simply because it’s late and I keep thinking about it…