I would really like to add my two cents into this subject. As a hypothetical situation here. My brother has an American Bulldog who is the sweetest big lug around. If I were at a party at my brothers house, I would never dream of pulling a prank like that when there is a potential that the dog is around. Very simple: Even though the dog is sweet and wonderful and is great around the kids, I don't know how he would react to a threat to the kids even though he does know me.
Pure and simple: Dogs and potential threats to their family do NOT mix!!!!
People here are blaming the breed, the breeder, the owners along with training. Basically, unless you have trained your dog to be an attacker, none of us really know how any of our dogs will react to a threat of that nature. If someone I know comes into a party in my back yard and creates an atmosphere where the kids are running scared, I am completely clueless as to how my dog would react. Would she run with the kids, hide in the corner, become agressive or attack. I don't really know. I do know that I have been working very hard to make sure that my dog reacts in a friendly manner to people in general, but not to react if attacked.
It would be interesting to know how many people are really confident that their dogs would not react in a negative fashion to that kind of "perceived" threat.