How old is the kitten? If its still so young that it needs non-clumping litter then the only thing I can think of is to put the kittens box somewhere that the cat is too big to get into, and the cats somewhere that the kitten can't get up to.
The kittens would be easier: if you buy a hooded box, you could turn it around so the opening isn't acessible and then cut a tiny hole in the side that is just big enough for the kitten. The cat, I don't know anything about where you live, but could you put it up somewhere the kitten is still too little to jump up on?
Zissou has used Fresh Step clumping clay litter since she was two months with no problems at all. Never tried to eat it or anything.