Washington Post: First lady Michelle Obama stepped into a more prominent, more ambitious role on the world stage this week with visits to Haiti and Mexico City. Her overarching message: Children can change the world. And she wants to make sure they have every opportunity to do so. Obama chose the Mexican capital as the launching pad for her international agenda of youth engagement. She arrived here at Benito Juarez International Airport on Tuesday evening, where she was greeted by a small clutch of dignitaries, including Carlos Pascual, the U.S. ambassador to Mexico, as well as the Mexican ambassador to the United States, Arturo Sarukhan.