Is he neutered? This might help if not, but really it sounds like dominance to me. She is going to have to put him down with the help of an adult if she is too little to do it herself. Ty has to see her as a leader or he will not stop. Maybe you can leash him and she can control him better with a leash or maybe she can carry small cookies and make him go into a sit/stay and reward with a treat when he listens. Good luck.