I don't think pitbull owners understand what these dogs are. If they did they wouldn't even contemplate leaving them around children. They descend from dogs strong enough to bait and fight bulls and bears. Additionally they are terriers. Do you know what a terrier is? Terriers are bred specifically to be killing machines.
Look at these little motherfukkers catching over 300 bodies in one afternoon. Now imagine an animal strong enough to fight bulls with that same killing instinct. No matter how well it's trained, they only need to snap once.