Most bull mastiffs I've met have been pretty gentle but you never know when any dog might suddenly...snap. Especially large dogs.
There's always a small possibility that the kids might have done something that triggered for the dog to do that.
And it's hard to tell if a dog is like that if they've always seemed calm and gentle I woudl just think it'd be paying close attention to their behavior changes if it was one you lived with.
I have a saintbernard who is a inside dog. She's never bitten anyone and rarely barks. She's calm and acts like a mother to all of our other dogs and cats (she's also like 5 times bigger than any other animal we have). We've had her for a few years but still we're careful because if that calm nature ever changed and she did suddenly turn we wouldnt really be able to do anything about it because my dad cant even keep her from running on a leash (he's about 6ft and gets dragged xD It's actually fun to see )
Hearing about dogs doing this kind of thing is sad and in a way scary especially around strange dogs. Alot of the dog attacks are provoked in some way.
I still <3 dogs though...