Their appearance will be a rare one: unlike in earlier political races, they have barely been on the campaign trail this time. They have spent this summer swimming, playing sports and attending camp in New Hampshire. They are no longer the small children who toted little pink bags with Uno cards and markers at campaign events years ago in Illinois and Iowa. Malia, now 14 and nearly as tall as her parents, is a varsity tennis player with a cellphone. Sasha, 11, who seemed to grow overnight this summer, can chat in Mandarin.