Reading about the Australian Shepard that protected the family child put me in mind of a Collie that we had when my daughter was about 5 years old. Her playmates would come over to play with her and the collie would ride herd on all of them. If they headed for the street, he would herd them back into the yard. If someone came up to them, he would get between the children and the adult.
Even the parents would have to call before they came to get their child. The dog never showed any signs of agression, other than if a stranger came up to one of the children. As soon as a family member came out, the dog was fine with letting anyone into the yard, whether to pick up thier child or to come into the house. He would bark if someone rang the doorbell, but when we opened the door, he was fine.
Several parents told us that they never worried about their kids when they were playing at our house. Not that I didn't have some liability worries but my wife never worried about it.
I have had dogs all of my life, from raising pure bred labradors when I was in middle school to owning everything from a St. Bernard, Poodle, Cocker Spaniel, Shepherd/Doberman, Rottweiler Shepherd, Collie and an American Standard Terrier/ Viszla mix (The absolute sweetest dog that I have owned) so I understand agression in an animal and train it out of them when they are puppies. My dogs were always trained from puppy hood to be people friendly and I have never had a problem (knock on wood). Never the less, I still kept an eye on the more aggressive breeds when they were around strangers.
It has been my observation that the most serious problem with dogs tends to be their owners.