Frontier Airlines and other ultra-low-cost carriers have been soaring to financial success by dropping ticket prices and charging fees for such formerly free services as seat selection. But a new push by U.S. Sen. Michael Bennet of Colorado could stall those efforts. Bennet, a Democrat, co-sponsored a successful amendment to the Federal Aviation Administration reauthorization bill this week that requires airlines to allow parents to sit next to their children on a flight at no additional cost.