It must be a really tough position to be in, and not one that many would understand.
Whilst it's true that any dog can attack and cause serious injury, some breeds have been specifically bred for aggression and traits that make them good fighting/hunting dogs. These same traits mean that they generally do not make good family pets - there are exceptions to every rule, but for the most part the dog has a deeply ingrained instinct that is difficult to overcome. Most dogs when provoked to the point of biting will snap then release. Enough to get the result they're after, the cessation of whatever behaviour is distressing the dog.
Some breeds though, do not release - and a pitbull is one of these. They will keep going. And going. And it's that that causes the severity of the injuries seen. Personally, that's not a risk I'd be willing to accept around young children. Simply because no matter how much supervision there is, and how well trained the dog, there is always the risk of a kid finding that one thing that triggers an aggressive response.