Suerte,
Sorry if I read your post wrong.
It seems that many people have the misunderstanding that an overtly "protective" dog is a good thing around kids. Generally the false conception is that these dogs act out to protect the family they feel a part of. Overt agression like this is more comonly a sign that either 1) the dog thinks the kids are its own property or 2) the dog has a weak temperament.
Dominance issues can be controlled to some extent with agressive training, but dogs that tend to attempt dominance over humans are always a risk when unattended off lead.
Dogs that have weak temperaments that also show agression tend to be "fear biters." These are the worst kind. They need lots of care, and loving, quiet environments unless they can somehow develop some confidence and evenness of temperament. Not likely. Otherwise they should be put down. They are bombs waiting to go off.
From the description you gave, I couldn't help but envision one of those two types.
You are right that selfless protection is a good quality in a dog. I own a German Shepherd because he has that instinct and is extremely smart. But you wouldn't know he is so protective if you met him -- he is a greeter and a wagger, a real lover. Make no mistake though, he will absolutely not hesitate to lay down his life to protect me. He is very good at percieving what is and is not a threat.
A dog that displays combative agression without much provacation, or one who is hesitant to socialize, is one to watch out for. Those are the dogs that wreck it for the rest of us working breed owners. They percieve everything as a threat, and act accordingly. Whether the master's or the dog's fault, those dogs need to be kept on lead or penned at all times. They are not dogs you want to own or have in your neighborhood.
Scott
PS: I should add that it can be nearly impossible to train those problem dogs. They will never fully trust you enough to listen, and they will never have enough confidence to apply what they learn. They are basically worthless dogs.
There used to be a practice of training a dog for combat by agitating it. The general theory was that everyone (in the military unit) would beat the dog except its master. They basically took good dogs and drove them crazy, essentially ruined them. The dogs learned that fighting was the only way to survive. Not a good way to treat an animal, or to get what you want, a trained protection dog.
Who said anything about parks? Why wouldn't someone want a dog that would defend children in their territory from stray, wandering dogs? Selfless protection of their master is a good quality in a dog.