
I have a 7month old BMD and his mum who is 3 years old.
My pup does alot of play fighting with his mum, and now he is bigger then her!!
Play fighting is great fun for both dogs, and if one does yelp out then it is just a way the dog tells the other 'that hurt!' your too rough, then if all is ok they will play again
Play fighting is a good way for your younger puppy to learn about doggie language and helps his socialiseation skills.
I would not get involved with their play fighting unless matters turned nasty and aggression took over, but a very much doubt that would happen as all dogs play fight.
I have never had to get involved with my two when they are playing.