I have say that I feel this is nothing more then missbehaviour and they need to be stopped from doing this so it doesn't get more aggressive. A mother cat quickly punishishes her kittens if they start biting too hard. Just like children, kittens and pups need to be taught what is acceptable behaviour. As in small puppies the bites start out as play but as they get older their play can get rougher and the bites more serious, same in small kittens. It is up to you how far you want to let this behaviour go. If you don't want it to go too far.... when the kitten starts to bite your arm and you don't it to, pick it up saying a firm "NO" and place the kitten on the floor or away from you. This will be a negative action for the kitten as it loves to be with you. Won't take long for the kitten to figure out that biting will cause it to be placed away from you.