Under pressure from health and children's advocacy groups, McDonald's Corp. is making changes to its famed Happy Meals. The fast food chain will add a serving of fruit or vegetable to all of the meals, which are aimed at children, and shrink the portion of French fries. The changes, to be announced Tuesday, will take effect in September in some markets and then roll out to all 14,000 McDonald's restaurants in the U.S. by April. McDonald's said it first experimented with cutting fries entirely from the Happy Meals, but children and parents rebelled. "People come to McDonald's and, first of...