My kitty Hannah does that same exact thing to Squeekers. I got worried at first, but the I realized she was just grooming him. I think that I would just watch extra carefully, just to make sure its play/grooming. I always thought that when a mother cat carries her babies, it had to hurt, but it doesnt. Maybe it is just an instinct that the cat/kitten knows but we dont? So I think it is probably pretty normal.