Armed guards usually only end up being the first victims... the attacker always has the drop... you would need barbed wire and blast zones to keep children truly free from danger... one armed guard only really gives a false sense of security.
I thought of that also. One armed guard can get the drop on a perp, if the perp isn't aware of the guards location, or doesn't plan to take him out. I agree that a very well thought out plan would include taking out the guard, but many of these incidents are not very well thought out. Only a few were. Therefore, it is my opinion that an armed guard, as you stated, gives an appearance of added security that may thwart some attackers.