From the New York Times’ Well blog:
With Cheerios and other processed cereals, “you basically have rapidly digested sugar mixed with bran and germ,” said Dr. Dariush Mozaffarian, dean of the Friedman School of Nutrition Science and Policy at Tufts University. “It provides fiber and minerals, but also digests in the mouth almost immediately.”
That gives you a quick spike in blood sugar, but no energy for later.
Basically, if you’re going to eat (or feed to your kids) processed cereals, you should add fiber and fat to slow down the digestion of those carbohydrates so they’ll have enough energy later on.