They don't begin using the litterbox until about 4 weeks, but at that time I like to use World's Best (corn based) or Swheat (wheat based). They clump and are totally safe if (okay, when) the kittens try to eat them.
They are a little pricey, though. If cost is an issue, you can also use plain clay. Pellet type litter (pine or newspaper) is also acceptable, but sometimes kittens don't "get" these types right away.
The only kind that really isn't safe for teeny-tinies is clumping clay.
Whatever kind you use, make sure you use just a shallow layer of litter so the kittens can keep their footing in the box.