My kitty Mishka does this when he is feeling extra, extra affectionate (usually about 3am) he will purr and lick me and every now and then give me a gentle bite. I call them love bites - they usually very gentle, he just kind of "gums" me. I think it's just another sign of affection. If that's what your kitties do, it's okay- just make sure that you show them it's not okay do do it harder. A loud NO and stopping your "lovies" session will usually do the trick!