it's a high chance they are playing........ don't worry cats know how to ward off other cats if they are fighting.....at least most cats do....... if any one of ur cats end up getting hurt or bruised and all.... hen yes they maybe fighting......other wise i think it's best to let them enjoy.....
You can know if your kittens are playing or fighting by looking out for physical injuries in either of them. While playing, they do not bite each other hard, but while fighting the bites are deep and they would make a lot of noise.
If there fighting then there scratching each other and it hurts so they start to make noises and they might start bleeding.If they are playing there still scratching each other but in a playful way and there NOT getting hurt.There also might be biting each other SOFTLY.Fights usally happen with cats with a boy & a boy and it usally over a gurl or they might not like each other.Its a bad chance it would be a gurl & a gurl.But it wont defently not happen with a boy & a gurl.If it does ut means there mating.
Kittens like any baby predatory animal 'play fight' .. that is an instinctual behaviour to help them to hone their fighting skills to mate and catch prey and is a good form of exercise to help build muscle and strength.