I have gone thru this many times with my son. The letter needs to come from the parent who is not taking the child. For example I have custody and my current husband and I took our son on a cruise. My ex had to provide me with the letter. I did not have to be present to sign only my ex. Now this also applied when I took our daughter to MX, it was just her and I and my husband had to also provide a notorized letter that it was ok for her to travel with me.