The other problem is that so many people forget that dogs are ANIMALS. Any animal can attack...ANY one can. It's far less likely with some, far more with others, but the flat out fact is that animals have triggers, just like people do.
With big dogs, especially big aggressive dogs, you have to firstly have a pack leader that can enforce pack discipline...and that leader has to make it clear to the dog that all humans are higher ranking than the dog. This isn't all that hard to do, but many owners of Pits, Rotts, Dobies, Shepherds and other big dogs do a very poor job of it. Many dog owners are stupid about the things that will trigger an attack...and many parents haven't taught their children how to act around big dogs. It's a horrible combination of stupidity...and the children are the ones that pay the price.
We currently have 2 Pits. One my daughter bought as an adult, and he is chained inside a pen to make sure he can't get to anyone and the kids can't get to him. He is perfectly fine with all adults, but we don't know about kids and small animals, so he just doesn't have the opportunity to do the wrong thing.
The second was raised in the house. He's been taught all his life that he is lowest-ranking critter in the house. Our old female Shih Tzu snaps at him when he gets too rough, and he backs down. Even with him, though, we don't leave him alone with the small kids. And his food and toys are in his crate, not out where the kids can get them. I'm no kind of perfect dog owner...we messed up with the first Pit my daughter brought home and he made my little Scottie into a pin cushion. (Scottie lived, Pit was put down...no second chances.) We're more careful this time.
