My mother and I will be cruising with my nieces (her granddaughters) as a graduation celebration in June on the Celebrity Beyond. At the time of the cruise, one will be 18 and the other 17, so they each are sharing a room with one of us since they are under 21. Being that we are not their guardians, is there any other paperwork/documentation we should have besides passports? Will there be any issue with both of them getting on/off the ship with just me if my mom decides to stay onboard in port?