The type of litter can be a reason. First try switching to Purina brand(worked best out of all I tried) unscented litter. Buy the small sandy one. Place the food bowl far away from the litter box. Make sure the litter box is big enough and easily accessible by the kittens. If they refuse to use it, pick up their feces and dump them in the litter box and they will eventually get the point. Don't dump too much in there since cats would refuse to use a dirty-smelly litter box. Clean the floor where they pooped on.