Two dozen San Francisco schoolkids sporting white turtlenecks and Santa hats got a very un-Christmaslike civics lesson the other day when they showed up at Union Square hoping to delight Christmas shoppers with holiday carols. "They just wanted to set up next to the Christmas tree and sing,'' said Donna Vargas, one of the parents who escorted the fifth- and sixth-graders from San Francisco Day School on Friday's outing. Instead, they got the boot. Seems they didn't have a city permit -- so after a brief run-in with the park's security, the kids were shooed away. "How can children not...