It sounds like the dog was just taken over, without having been subjected to obedience training to break any bad habits acquired from the previous owner. In addition, the dog was left alone with small children. On top of that, it wasn’t properly restrained. Finally, the question is whether the grandchildren were taught about how to interact with dogs, i.e. to avoid things that make even the ones docile vis-a-vis humans uncomfortable. These factors, in combination, were a ticking time bomb, even if you leave out the demonstrated human aggression (i.e. the dog going after the kids when muzzled) and potential breed issues.
You will note the term in the translated text “Freedom driver”. This is a person who is part of the transfer of pit bulls in American animal shelters due to be euthanized, which are then picked up and transferred in vans, several to a van, over the Canadian border to Canada.