Interesting conversation. This is the way I look at it. Some people might get upset, but I'm just being honest. I would not personally adopt 'powerful' dogs from a shelter if I had young children. When I say 'powerful' I just mean a dog that has a very strong bite force. Simply because you don't know the exact history and because of the potential damage it could do. Again that's me and just the way I see it. You just don't know. If the dog doesn't have the capacity to due tremendous damage than that's another story.
Not to mention the fact that even a dog who is a sweetheart could very easily accidentally hurt a child. I agree with you completely, introducing a fully grown dog to children isn't the best idea, even if it is due to no malice or ill will of the dog the potential for the child to get hurt is a very real thing that must be considered. Take my uncle's massive St. Bernard, it LOVES people, and it's favorite thing to do is plop down right on top of you when you're sleeping or sitting and refuse to move. At 6' and 180lbs that's not such a big deal, but a 4' kid weighing 80lbs could easily suffocate or be hurt by the shear weight of his fat behind.
POINT: big dogs who are used to adults have the potential to harm young children with actions that would pose no hark to a fully grown individual and this needs to be taken into consideration