It teaches them that things are important only to the extent that they touch their personal world. It also feeds the parochial prejudice of American civic religion. American history has traditionally suffered from an exaggerated sense of self-importance. This is not to deny that God has bestowed a number of blessings on America. As those who founded this country acted in obedience to God, He honored their obedience and the nation has prospered.
Blessings always follow obedience.
But God promises to honor all nations who obey Him. Those who assert that God has some special relationship with the United States of America that He is not willing to have with any nation that obeys Him skate dangerously close to the heresy of proclaiming that the United States is the new Israel. It is important to teach children that God is the author and director of man's history. This applies to all of history, not just American. History is not just a series of random happenings.