Of many dogs in my life, the only guarder has been the cocker. And I mean serious , potentially dangerous guarding. COL saved his life with advice and comfort for me. I raised the question here last year on whether it was a mental problem like autism, because the dogs I've read about , like mine, had no obvious trigger. The tv dog trainer Victoria someone, shocked us all when she advised an owner to have their guarding cocker PTS. You CAN modify the circumstances at home, your own handling of the situation, distracting the dog, ignoring its antics like I did. However, as you know, you can't expect visitors and children to understand. You must have rules.
My mantra everyday was 'Don't stare at him in the eye, don't leave anything on the floor, don't escalate the situation. Walk away or turn sideways, and they will Give up.
Once the adrenalin is raised,very fast like this, they are beyond reason. Let them have space to back out of their own trap.
Despite this, my boy was the love of my life, and gave love back in barrow loads.
I emailed TopBarks, I'm sure he'll still help. A good trainer is worth the money. I am using one for another dog, and the cost of seeing him for an hour a month is less than weekly group lessons, and it's all about you.