Make sure you have notorized permission for them to travel with you and a notorized medical form giving you the right to seek medical care and make decisions in an emergency. The can walk-in to any Passport Issueing Office and get it within 2-hours on the spot. I got mine in Miami before a flight to Europe, which is an example that her parents can go to any office. Just because her parents go with you to check-in doesn't mean they will let her board if they state all minors need passports. Passports are needed to RETURN to the US and not leave the US. If any of you should break a leg and/or need hospital care and/or miss the ship in a port, you can't return to the US without a Passport. You would then be forced to contact a US Embassy to obtain one, which can take several days, leaving you with added hotel expenses. Be smart and get the passport. No judgement, but traveling with 15-year old boyfriend and girlfriend is playing with fire. You can't watch them all the time.