So now we're down to concealed carry being a bad idea everywhere? Because what you just described is what's going on OUTSIDE of schools.
And as far as "6 year old Amy" is concerned, I think she'll be fine as long as I'm not aiming for the shooter's knee caps. Unless he happens to be 6 years old also and the same height as all the kids, I'm aiming for chest and head...well above any kids.
But you ask if I could live with that kind of mistake. I guess my over analytical mind thinks of it this way: I've got 2 options, do something or do nothing. By doing something, yes there is a chance of making a fatal mistake. But I can't imagine watching Adam Lanza open fire on a bunch of first graders, having a shot on but refusing to take it because I was more concerned about the legal implication, should I miss. Guess what, that shooter isn't worried about any of the rules of humanity or rules of engagement. So then there's the option of doing nothing. There is a very high probability this option will lead to your ultimate demise, but lets say you lived through it. Would you be able to live with yourself any better knowing had the laws have been different, you could have had the power to do something about it but instead you were just as helpless as all the kids?
Most cops (the trained personnel who I'm guessing are the ones you are saying should be allowed to carry in schools) who are put in a situation where they must use deadly force generally hit their mark 25-30% of the time. I'm sure civilians are lower, but I'm willing to bet it's not so much lower that you could justify them carrying in schools over us.