Staffies were bred to bite the noses of bulls in a baiting ring and hang on with all their might. Whatever the temperament of an individual dog, it has an innate prey drive, which has been selectively bred over the generations to strengthen whatever part of that drive suited the breed's purpose - eg chasing, biting, fetching. Staffies were bred to bite, that's why their jaws are strong. Sorry if you don't like that and love your own staffie
This is true but it doesn't explain the full picture. Staffies gained their reputation of being great with people and children because of this selective breeding.
Bull baiting and fighting breeds needed to be handled by people (vets/owners/referees) when they were in a highly aroused state, a state in which many, if not most dogs, would bite.
Because of this SBTs who showed any sign of aggression towards a person was not bred from. This breeding went on for decades. That's why SBT are regarded as bombproof. They have been bred to be accepting of human touch no matter how aroused they are.
What happened here was solely the fault of whoever decided to leave the baby on the floor with two unsupervised animals. It could have happened with any breed and has in fact happened recently with JRTs, Northern breeds and Mastiffs.