To be fair the story has been updated since I first posted (i'e the police statement). As I said I have NO experience with urban foxes.
I do have plenty of experience with country ones. The 2002 report of a fox "darting" in the house and grabbing the child "by the head" does seem consistent (that is how they will grab young livestock - by the head)
Foxes I know of do not go hungry, they move on to more plentiful food sources. Maybe that validates this story, I can't help thinking cunning Mr Fox will find easier opportunities (bins etc) in East London than a pair of twins in bed though. Foxes are actually pretty small and it must have caused quite a commotion. Why didn't the parents kill it? Sounds harsh but thats what would happen if it was attacking my daughters and IME it's the only way to stop it in a frenzy.
They used to hunt with packs of dogs becouse the hounds are basically thick as the preverbial, one would be no good, lose the scent or just basically lose interest. The fox would run, literally for it's life, when cornered it would fight but be lucky to "take chunks" out of the dogs before it was, as TAFKASTR rightly says be inhumanely torn to pieces.
Maybe I'm being naive but this just doesn't seem right.