This story is full of red flags!!
First of all, no rescue organization should have allowed such a large (and strong) dog to be re-homed by a family with small a child...
....living in a flat is another!!
Usually there is a good reason dogs to end up at rescue places and majority of the time the issue is to do with the temperament of the dog..
The child being allowed to play with such a strong dog, rescued only few weeks ago is another red flag!!
Cant believe such a tragedy has been allowed to happen in UK due to negligence!! They are usually good at handling safety issues..