At what age is it too early to make kids begin to appreciate that they live in a (still somewhat) free country? Apparently kindergarten or first grade are not too early to begin teaching them that some men put their part "P" in another man's part "A".
Kindergarten 'graduations' are for the parents but also for the kids who are shown that there is recognition for achievement. At that age, completing an entire year of even simple schooling is an achievement. There is no reason those kids could not and should not have sung the song. That principal out to be making up cardboard boxes in a factory someplace where she cannot harm our kids.