This subject has been raised distressingly often - because the instances of child deaths like this are rising.
As most people who know anything about dogs agree, any dog, of any size, breed, or temprement, can turn vicious without warning or apparent provocation - usually because pack hierarchy is involved.
This sadly means that new arrivals in the family are seen by some dogs as a threat to their position, and their instinct is to remove the threat - and when the dog in question is a banned breed, this is often the result.
"Why would anyone allow one of these killing machines anywhere near children?"
Because the owner believes their dog 'wouldn't hurt a fly ...' which is arrant nonsense because any and all dogs will attack if the circumstances dictate - and that is often something known only to the dog, who shows no advance warning of what is going to happen.
A simplre rule is - children first - dog after. Dog first children after is setting up this situation, and it's too late to worry afterwards.