I have never shown any ID for my kids when they travelled domestically. They both have passports, but I do not bring them with me unless we are going somewhere international. Usually the TSA agent will ask them their birthdate and name, which is visible on the ticket to be confirmed by the agent. The airline check-in desk specifically says that they only want the adult's IDs. I have travelled several different airlines with my kids throughout the years--mostly post 9/11 (United, Delta, Frontier, Allegiant, now defunct-US Airways, BMI and Continental).