Is my cat hurting my kitten? please help!?

my girl cat (Tilly) is 6months and my boy kitten (Biggles) is 9weeks (very small still) he has been with us 6days & tilly hated him at first but after 2days they started play fighting etc.. but it gets a bit rough sometimes (well sounds rough) she will bite his neck and tackle him and he will be crying in a fighting way and so i pull her off him and tell her off a little but then he runs back to her for more! so im not sure if he is yowling because of pain or as part of the play. could she seriously hurt him? she is just playing with him but it does look a bit rough sometimes but since he runs back for more i feel i might be telling her off for no reason and i feel bad. we don't smack her we just yell her name or stomp and they stop and she runs away.