First a disclaimer: I have a 70-pound pitbull and I have a 120-pound American Bulldog. I love them dearly, and they love and obey me in return, but I spend a lot of time with them to remind them how to behave and communicate to them what I want them to do. They very much want to please me and protect me, but since they don’t speak English, I need to figure out how to communicate the rules of our pack to them. I have had to spend extra effort to make sure they know that they cannot push my kids around (who are adult-sized teenagers) and that commands mean the same whether uttered by me or by the kids. I also have had to spend a lot of time with m kids so they learn how to communicate with dogs.
Pit bulls tend to be muscular and active dogs. If you fail to train them properly they can do a great deal of damage to a small person simply out of a combination of exuberance and lack of knowledge of children as being more fragile than adults. In addition, because kids are small, any dog needs extra training to be certain it understands that the child is an extension of the adults, aka the pack leaders, and not rivals to be put in their places.
Consider how irritating a badly trained chihuahua is and what damage it can cause. Then put a large, active dog breed into the same household and a tragedy can easily result. In any case, the family dog should never be left alone with a small child, ever. Far too many people are themselves untrained in training dogs and they end up with badly behaved animals as a result. Those are the dogs that can be walking time bombs.