I read through this entire thread before offering up my 2 pennies.
I own 2 Belgian Malinois, am a grandmother, and have owned German Sheps and Dachsies before. I train my own dogs, and really know a lot about dogs. I do not believe that specific breeds are dangerous, but any individual dog in any breed can be dangerous.
Size is the factor that makes a dog a "killer", not breed. Poor training and socializing is a factor in dog biting and agressive behavior, as is poor dog manangement.
My Belgians are as sweet as pie with me, but my male acts like Cujo on steroids when a stranger comes to the door. I have him trained to do this, because we are elderly and live way out in the boonies. But, I also have him under control at all times, and as soon as I tell him "enough", he sits and waits for me to go to the door to see who the stranger might be.
He does not bark this way this for "friends" who come to the door.
All that aside, I would not let my grandchildren play with either dog unattended. When they visit, my dogs are leashed and under control at all times.
Responsible dog owners must understand the pack order, what environmental triggers will set a dog "off", and at all times keep a dog under control. Things that would set my well-bred, and well-tempered Mals off, you might wonder. I know, because I have studied the breed, and know what "herding" dogs have been selectively bred to do.
For example, small screaming and running 3 or 4 year olds might trigger the "prey" instinct. A flying crow or owl overhead might trigger the "attack" instinct. Strangers coming into our dogs' "territory" will trigger the "defend the homestead from rustlers" instinct.
The grandparents, in this case, should have known enough about their Rottie to not allow him unleashed around their daughter and the baby, in that specific circumstance. The daughter should not have had the baby in her arms, with a hose in her hand, with the Rottie loose in the yard. Too many triggers were potentially being pulled in those circumstances.
My female Malinois is "hose happy". She thinks the hose, when on one certain setting, is "attackable". We use this to give her some fun and exercise, but I wouldn't suggest that my daughter-in-law use the hose with a baby in her arms if I weren't right there beside her, for who knows what goes through a dog's brain in those circumstances...
Knowing how ready to go on the attack my male when crows are outside threatening our "Air-Sheep", I would not let a casual friend stand between my male and the door to the outside, for my male does not know that the casual frien will immediately let him out to give chase the same way he knows we will. My male could easily conclude that the casual friend was in the way between him and the varmints, and snap at him to get him out of the way.
Just because a dog learns a command in the house, he does not generalize it to other locations. Just because my Mal knows to "sit" on command at the front door, doesn't mean he knows the command at the back door, or out in the yard, for that matter. He has to be trained to do so in every location possible, so he doesn't think "sit" only means it when at the front door.
What I am trying to explain by this long post is that people are responsible for understanding their pets temperments, making good judgements about how and when to control their pets, and to be constantly training and re-training them. My preferred means of control in a good leash, a good set of commands, constantly practiced oin all location, and when I am not able to be paying attention to the dogs, good strong crates for them.
My dogs will go to their crates on command when we have strangers in the house, as they like their crates and feel safe in them. The reason for this is that I always feed them in their crates, do not use them except in certain circumstances, such as a stranger in the house doing odd things, like plumbing, working on the air-conditioner or furnace, with strange and "dangerously scary" tools, etc...
People need to spend more time thinking about what dogs need, and some people shouldn't have big dogs at all. They can mess up the little ones, and while bites will still occur, at least people won't get killed because of owner stupidity.