I always make the parents write a letter before I take kids on vacation. I even do it for my nieces that have a different last name. You never know if one of the airline employees might want proof that you are allowed to take someone else's child to another country. Immigration could also be an issue. I have never had it happen, but it could really put a damper on your vacation if it did. Included in the letter, I have them give me permission to make medical decisions on behalf of the child in case of emergency.
We always have our kids bring friends with us on vacation so that they have somebody to hang out with. So every year I gather all of the passports, photocopies of the passports, all of the confirmations for car rental, outings, etc, the letters from the parents and any other important trip-related documents and put them into a folder. I impress upon the whole family and the friends of the importance of the folder and how our vacation could be ruined! if the folder gets lost. Guess who left the folder on the floor at Hadicurari???
Our room at the Surf Club wasn't ready last year on arrival so we went next door for lunch. After all of my lecturing about the importance of the folder, I left it sitting on the floor! Thank goodness they picked it up and put it safely away. It was actually a few hours before I even realized it was gone. I took quite a bit of ribbing for that one!