if you're not seeing fur flying and neither kitten is getting injured (scratches, punctures from bites, etc), then i'd keep an eye on them but let them play/wrestle.
if one kitten starts avoiding or running and hiding (like s/he is afraid of the other kitten), then the other kitten might be playing too rough. clapping your hands should cause the kittens to pause in their wrestling, and you can then pick up the too rough one. you could give the too rough playing kitten a "time out" -- place kitty in a room by him/herself for 10 minutes, then let kitty out again. make sure there is a litter box in the time out room though -- kittens many times don't realize they need to pee/poop until they have to "go" right now.
"Rough" play is normal. It seems rougher to us because there's lots of biting going on which humans are uncomfy with. They can Usually get the message across to each other when enough is enough. As long as there's no real injuries and no aggressive body language (ears back, hair raised, etc. ) just let them be cats. If you see one cat hiding/scared of the other cat when they're not playing, then you know something different is going on and then you should address it.