I personally think if you've got a baby, you shouldn't have a dog like that as a pet. Just my opinion, I know there's a lot of dog lovers. But pitbulls (and maybe not just pitbulls - rottweilers too), no matter how docile, still have the capacity to turn in an instant. You always hear the same thing after things like this - 'He's never done anything like this before' - exactly. It's only happens once, and then it's too late. There is no warning with these animals. The risk having an animal like that living in the same home as a small baby or child would be too great a risk for me to take.
R.I.P. that little girl. What a tragic, horrible waste of a little baby's life.