The first time I was exposed to the Pledge was in kindergarten. I had no idea what the words meant.
As this piece at Reason tells it, it was a late 19th Century response to large waves of immigration.
It's not really about a flag. It's about "the Republic for which it stands." It's a way of testifying that you agree to the American social contract. In other words, "I agree to this deal."
Kids cannot know what that deal is. It's quite a serious deal, really, because it can cost you your life, plenty of your property, and some of your freedom. It's freely given, too, if you're old enough to give it. You can always move somewhere else. Tom Friedman can move to China.
I don't care whether Rousseau's social contract theory is valid or not; it's an essential concept anyway because it makes it clear that a conscious choice of citizenship is meaningful and important - even if secular and not sacred.