The first, obvious, difference between a mastiff or any large breed and the small ones is that an attack from a big one is more likely to be fatal. The second is that mastiffs were not originally bred as lap dogs or house pets; they were bred for protecting property or people or for fighting other dogs. This element will persist, regardless of how much they are bred for show; they have a stronger sense of dominance, and are equipped with the jaws and muscles to assert it.
Wolfhounds were not, but I would never let mine in company of a child whether alone or as a group. It is sheer madness to do so, because you never know what the child may do, innocently or not, which will trigger the attack response. Having seen two of mine fighting once, you wouldn't want any child involved in that aggression, though normally they are as mild and as friendly to each other as kittens, and are always utterly friendly to humans; they can do serious damage, but, being evenly matched, no real harm comes of it.
And keeping a big dog in a small flat is never going to be conducive to it being calm.