I have never had success in keeping a kitten from playing it the litter box. I hope someone here can give you a solution, but if they can’t, here is what I do to lessen the problem. I keep the box super clean. I make sure that there is something under the box to catch the litter that is being scattered (an old plastic mat that you use under office chairs works really well). I keep cat toys about 3 feet away from the box, close enough that the kitten will hopefully get distracted by them and start playing with them instead, but far enough away that they don’t end up in the litter. I keep wet wipes on hand so that if they happen to get any feces on them, I can quickly wipe it off (of course, you need to be there for that to work). I have also found that having a litter box with a lid, lessens the problem, because they don’t seem to like to play in it as much, if it has a lid (but that may just be the kittens I’ve had). If it is any comfort, every cat I have ever had, has stopped this behaviour on their own as they have gotten older.