If we still had school-age kids yes I would probably keep them home, and for two big reasons. One, 9/11 is one of a few significant days for terrorists and they like to strike repeatedly on days that have significance for them. They have already said they will. To them it makes a statement. Two, it has already been reported that they are looking at hitting schools, churches and malls and other "soft targets". Christian and Temple schools would be especially attractive targets. It wouldn't be worth it to me to take the chance.
Now if we had something planned that didn't take place in a highly visible public place, like a music lesson or baseball practice, then we would go to those activities
And no, I would not consider being prudent letting the terrorists win.