There are not a lot of children killed by chihuahuas, or any of the smaller breeds of dogs. They just don't grow large enough to do that kind of damage. No amount of training can bulk them up enough.
I suppose a greyhound would be big enough to pose a threat, but they are built for speed.
I've never seen a Labrador trained to kill, they have the mass but not the musculature. I have seen huskies and malamutes trained as guard dogs, and they can become very dangerous.
Training has a lot to do with it, but some breeds have the build and temperament to be more aggressive and dangerous. It's called evolution.