There are several points to be made here.
Firstly, the family had for whatever reason acquired this dog and rather than training and socialising it, had chained it up in the back yard, presumably encouraging it to be aggressive for security purposes. Then the grown-ups [if we may so loosley describe them] went out, leaving a 16 year old girl in charge.
She, preoccupied no doubt with the usual things 16 year old girls do, didn't notice her 7yr old sister carry the toddler out to the yard and approach the Rottie and offer him to the dog 'to meet each other'. The dog presumably thought it was being offered a meal. I hope the parents are prosecuted, not I suppose that that will help their daughters deal with their trauma, which must be massive. Imagine those two kinds having to live with that.
Secondly, Rotties are a known aggressive breed in most circumstances, but there are indeed extremely gentle strains as well as quite vicious strains. Having been bred for centuries as guard and attack dogs, it's not surprising that the aggressive genes can surface in these dogs - often with no warning at all. In addition they have a particular type of jaw which - like a bulldog's or a 'pit bull's' - will 'lock' which is to say clamp, and it's almost impossible to cause them to unclamp in the way you can with other dogs' eg by a blow or kick to the jaw joint on the side of the head. This does make them very dangerous as it makes it almost impossible to deal with any attacks.
It's very foolish to think these dogs are reliable even if they come from a gentle 'strain'. They have a history of 'turning' which means it's almost impossible to trust them completley, eg with other animals or children. I'm pretty terrified of them if I don't know them, even though I've known several 'softies' well and even dogsat for one of them! My own dog was once viciously attacked by one and I only got her free by getting it off with a pitchfork. One I knew well and had always considered really friendly, went for me when I bent down to speak to her master who was driving 'their' car. I would never have one.
Even if they can respond to firm handling and early training, who would want the risk? - given even the 'softies' have been known to turn, you'd need to be always in control - I want a dog I can leave with someone else if I'm out of the country, or ask someone to walk for me if I'm ill.
The breeding and keeping of Rotties should imo be controlled, ie licensed. And I think that any strain of Rotties from which any of the progeny has attacked a human, esp a child in this manner, should be put down - all of the family strain. Sorry but it's the only way to eradicate the rogue genes. Ideally, they would be a forbidden breed imo. There is a reason why it's always the same two or three breeds which give rise to these stories.
Sadly, too often the kind of people who want to own Rotties and pit bulls other dogs with that kind of reputation fo0r aggression are exactly the sort who will not train and socialise them in the right way. And while that is so, there will always be a problem.
Btw: Pit bulls *should* already be extinct in this country as they were all supposed to be neutered/castrated when the act was passed, and that was years ago. The police are, as in the matter of dog-theft, not taking this seriously. There are unmuzzled and un-neutered fighting dogs wandering round council estates, living with travellers, etc etc, all over the country, but the police are too busy harrassing the motorist to care.