I'm leaving a week from today and I've never travelled with children on a plane before....I know that my youngest daughter (who is 1) will need a birth certificate because we did not actually pay for a seat for her. The other two are both 9 and 7 and we paid for seats for them (obviously). So....since they don't have any sort of id to prove who they are, should I bring their birth certificates with me to the airport? Is there anything else that I will need other than birth certificates?