I have two Rotties. Brothers.
I love them with all my heart (well, apart from the bit that's kept for the GSD, DH, DDs, etc. etc. etc.), but I don't know if I'd have the breed again, then again I tend not to stick to the same breed as the ones I've had have all been lovely representatives of their breed and I'm wary of not getting another good one.
As others have said - they're big, though not the biggest breed, heavy, powerful, fast, and they guard their spot and their people. So saying, mine have always been friendly with visitors both in and out of the house, though I'd not recommend anyone breaking in or trying to hurt us.
Training - lots and lots and lots (and lots) of training. Kind, fair, and food orientated. Mine are trained to voice and hand signals. You have to be constantly consistent.
They will lock on to other dogs and people. It's never been the precursor to any aggression, but it would possibly look scary to someone else.
Body language - I've done more and more reading up on it. Definitely helps.
Vets - we've had two practices as we've moved house. Both lots have loved the Rotties. The last visit had the vet sat on the floor being cuddled by a Rot. One had a poorly leg (turned out to be a toe) and when manipulated into an odd position did give a low growl, with no lip lift, so since then I've always said use a muzzle, but so far the vets have chosen not to. They were both castrated the same day and the vets and nurses handled them without issue. I will, however, always suggest and it's on the notes, that they consider a muzzle if there's a chance what they need to do will cause any pain.
Power - incredibly powerful. One ran into my DH's knee and broke it. Then licked his head when he (DH) was on the ground. I don't let them sit on me as the weight is too much. Othe than the vet, only I'm allowed to sit on the floor with them. When they're both piling in for attention the power is incredible.
Exercise - they now run free in our field for three hours a day. Before that they were walked for 2 hours a day each. Separately as I was working on their training. I used to walk the two together but want them better trained before I do it now they're full grown: I'm pretty certain I can control them but I don't want to take any chances. This would be the same irrespective of the breed.
I wouldn't have a Rottie and small or young children. They're like children themselves in a way. We plan pretty much our entire lives around them. The GSD can come and go and is far easier in lots of respects; though an interesting dichotomy she's wary of strangers and had to be muzzled at a vet's due to sheer nervousness, whereas the Rotties aren't muzzled and love a stranger if there's a chance of a head rub.
So, sorry but that was a very long winded way of saying that in your situation I wouldn't have a Rottie.