Sorry for your bad incident. This can happen with any breed of dog. Not just pit bulls. Dogs might not be socialized properly, or they might not be bred properly. You can't guarantee that all dogs of any breed are stable.
Often before an attack their are warning signs. Usually it is not out of no where. While it probably doesn't matter to you at the time when you were a child in the park there are things to consider, more then just breed. As for breed was the dog even a pure bred Pit Bull? Were they socialize? What was their environment/training like? How were they bred? This dog also did give some warning, barking and such. Though it is not a proper temperament for the breed. It is the owners fault for having their dog loose, especially one which is aggressive running free around children.
I'm sorry for what you and your dog went through after the Staffy attack. What lead up to it......were they playing together (you said it turned suddenly, out of no where, unprovoked) then a fight started, was a strange dog that just ran over and attacked your dog, was a friends dog who was fine then grabbed your dog.... Dog aggression and human aggression are separate (though some dogs display both) there are different reasons why a dog might attack another. No matter breed, just like aggression towards a human, any breed could show aggression to another dog. The reasons why can be many and you have to look at the situation. If you take breed into account the Staffy was probably not "sckitzo". Pit bulls can be tenacious and very determined, just as much as smaller terrier breeds, but when the drive is towards another dog it is obviously not the greatest as you found out. Most terriers were bred to hunt, hold, more then likely kill the prey, to keep vermin down. A terrier killing a rat is really no different then your cat killing a mouse. Terriers do not give up easy, they are full of determination. Even smaller terriers that were not used for dog fighting can get into a fight at times, these can be very serious. A dog even friendly might not always get along with every dog they meet.
As for history, it is true they were bred for fighting. Though for those who want to point that out as something bad they shouldn't forget the good parts of their temperament. It is not grand that your dog was attacked by a Staffy nor that they were used for bloodsport. But they did have a stable temperament that flourished in most dogs. Not only true of APBTs, but I also hear of SBT being the same. Friendly towards people, loving of children, placed in the pram with the baby on the way back from a fight as not at all being unheard of. The dog was to be trusted with their family, normally social around other people, had to take medical care after a fight without biting the person trying to sew them up, ect. Because of how closely they worked with humans and some requirements of them, they came to be a loving, loyal type of dogs. Pit Bulls can take very bad treatment and still come out loving humans. Not all have this temperament though as we see, some are a danger to people around them. Even their own family. This happens with popularity because of poor breeding and irresponsible ownership.
I also wouldn't say that they are not bred or at least used for anything else. Sure I can look in my dogs pedigrees and see fighters, but I can also see dogs bred for conformation, hunting, weight pull, ect. There are also Pits used for search and rescue, in law enforcement, as service dogs, therapy dogs, ect, ect. In working situations and sporting/competitions too (agility, obedience, ect) their drive and determination is actually a very good thing to have. Along with the quick rate many can learn and will obey. That part of them might not make them good pets for everyone, at least those which posess a more working geared temperament, but that is true of many high drive breeds. These breeds can often find themselves in touble because of lack of responsible ownership. They are not kept contained, not given socialization, not given an outlet (like mental stimulation and exercise), their needs are not understood by the owner nor the traits they have.
Take a look at the pictures, the first two are of my pittie, Budman who we found a new home because of his hyperness, but he is very lovable and a great dog, very gentle, too! The third picture is of my friend's beautiful and loving pittie, Blu.
My dog, Budman
That not a Pit Bull. If anything it is a mix with Pit Bull. I would say Pit Bull mix if you knew that one of the parents was a Pit Bull. Cute dog though, brindle is really a favorite of mine.
It looks like a shepherd mix, GSD perhaps. The coat type, tail and then the ears along with that are what lends the look of shepherd mix and not of a Pit Bull. Pits do not have that coat type.
Which is another problem with this breed. Identification if it is short haired and muscular its a Pit. If it's brindle its a Pit. If it has a red nose its a Pit.
Another issue is of mixes not being called so. For some it doesn't matter if it is a mix, it must be the Pit part that made the dog bad and attack. But for serious data it does matter how many pure breds attack vs how many mixes of that breed.
I also can't imagine getting rid of a dog because of their activity level - "hyperness".