I live right on the Canadian border and am also a children's lawyer and so I have some experience with this. You need a letter from the non-custodial parent IF you two have joint legal custody with you designated the primary residential custodian. That is to prevent kidnapping, as you can imagine. If you have sole, don't sweat it. You might want to bring a copy of your order of custody. The thing that I have found is that once the kid is about 14, with a school ID, the immigration nazis are a little more relaxed, as they can question the kid directly. The only problem there is if an agent looks your son in the eye and says "who is that man traveling with you?" and your son says "Um.. Not my real Dad, I don't see my real Dad too much since we moved away." CHECK PLEASE!
You absolutely need a notarized letter from the other child's parents- both of them, same letter. Make it as general as possible- don't spell out a specific port or precise time of travel in case your port gets switched on you. Also have them say EITHER you OR your husband OR both parties, in case you get separated.
The thing to keep in mind is that once these agents get started with their quizzing they don't want to stop, no matter how unreasonable they are and how obviously unlikely it is that you are kidnapping two teenagers via Carnival Cruise. Homeland Security is difficult enough to navigate at times, and I don't want to even think about what the Mexican authorities are like once they get their guard up!