The dog is deaf! The 'owner' has to learn how to live with a deaf dog.
It WILL be 'defensive' no matter what breed it is. This does not mean he is an aggressive dog.
A deaf dog can only cope with what it can see. Something coming up from behind or off to the side not within his sight they WILL be defensive.
No training is going to stop this because he can not deal with what he can not hear coming up around him out of his line of vision until it's smack in his face. Keep in mind a dogs hearing is one of the highest levels of his senses that he has lost.
Deaf dogs can be taught hand signals for day to day functions. But high activity around a deaf dog hand signals are not going to work unless you are attached to the dogs hip 24/7's.
The owner needs to find a group or class for learning how to live/train with a deaf dog. Owning a deaf dog is not something you can just write off and hope it works out well for the dog and/or family.
And when friends come over it is imperative that they are informed your pet is deaf.
And other peoples kids left alone around a deaf dog? Well that's a decision only you can make. I would not allow it because young kids brains are not evolved enough to take direction on how to act around the deaf dog and understand the seriousness of the situation.
Just saying, been there done that. I found finding a group or class for deaf dogs was the most important thing I did for my dog and family.