There are ways to safely secure a dog and/or keep a dog and child seperate without having one physically at your side. Baby gates, his crate, and brief periods hanging out in the backyard ensured that my dog was NEVER left to the whims of a toddler.
In this case this was an accident. The uncle moved in with the father of the little boy whom the father had just gotten back from foster care. The uncle had Rottie's which he kept kenneled at all times. The father and uncle put the 4 year old in front of the TV to watch his favorite video because they needed to attend to something in the cellar. While they are out there they decide to let the Rottie's out to stretch their legs knowing the child is in front of the TV watching his favorite show. For some reason the child needs his father walks outside sees the Rottie's are loose and tries to be brave but ends up being afraid and runs back to the house. This triggers predatory drift in the Rottie's a running prey object and they automatically go into attack mode.

My own Rottwieler has an extremely high prey drive which a Rottie is supposed to have and I've seen her go into predatory drift...she did it to me a few times when I first rescued her and she attacked me! I was walking her loose in a field and decided to run and looked at her and she lowered her head (eye stalk) then came running after me and attacked me quite seriously. This took a lot of positive redirectional training to modify. Anyway, having first hand experience with this enhanced drive with my own dog I "know" how easily and accident like this can happen. I know we all like to put all dog breeds on the same level and tell people a Rottie is no more dangerous than a Newfoundland. But, that is doing the breed an injustice and an owner who doesn't understand can be a dangerous owner especially if they don't do the proper socializing. Some owners are fortunate that they are able to socialize without even realizing they have been doing so. But, the owners of these Rotties obviously did not socialize therefore the Rotties never imprinted properly on children. Take for example a dog that is imprinted/socialized with sheep will not show predatory drift towards them. To bring up a good guardian dog you raise them with the sheep. To bring up a good herding dog you don't want them imprinting on the sheep so they will show proper predatory drift towards them. Different breeds have different motor patterns as well, a good sheep herding dog should not have finisher tendencies where a Rottie and terriers should have finisher motor patterns. That is how breeds were designed, the behavioral conformation formed their physical conformation etc.

Any who, this was an unfortunate accident that happened. I can't remember if any criminal charges were given or not now.