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.
I like just plain, old white clay litter for kittens myself ... but again, everyone's preference is different. I don't buy any that has fragrance, color, little crystals or any of that hoo-haw stuff, just the plain old Wal-Mart brand of clay litter.
I was using a clay litter when we brought Bijou home but he started sneezing and coughing so I changed to Swheat Scoop and I mix some World's Best with it. That stopped the sneezing and coughing.