Two things to try. First don't keep your cats food and litter in the same place. They should be a minium of 10ft apart, it has been proven that cats will not use a litter box that's near their food.
Next, find a neighbor or friend with a cat and 'borrow' some litter lumps. I know it sounds gross, but your kittens have fresh litter with no urine or fecal scent in it. They have no idea what to do. Put a urine and fecal lump in the litter box and put your kittens in the box one at a time. They will sniff it and immediately urinate or defecate to mark the spot with their scent. Then place the other cat in the box.
Leave the lumps in their for a day or so for them to go back to.
Conversely you can purchase litter attractment at Petsmart or Petco - but really you only use it once. And why pay for what your neighbor will happily let you take for free?