Staffies may be 'lovely' 'family dogs' to a point, but we had one when I was a child, and he certainly had his moments. He once bit my younger brother, who had perhaps been a bit rough in playing with him (but not very, he could only have been 4). He also twice bit a boy who played at ours regularly, because he caught me when playing kiss chase!
On a couple of occasions he got out and attacked sheep, and on another he killed a chicken. It sounds like he was poorly controlled, but these were all accidents.
As our Staffie got older and the family home broke up, he got aggressive with my dad, growling at him in defence, and would only respond to me. He also growled, hair on end, at my friends. He used to sleep on my feet, and if I got up to the loo in bed, would get nasty when I tried to get back in.
Don't get me wrong - I loved my dog, but there's no way I'd have one around my own DCs!