Senior Moderator; Moderator, Eco-Conscious Travel, United and Flyertalk Cares
Join Date: Jun 1999
Location: Fulltime travel/mostly Europe
Programs: UA 1.7 MM;; Accor & Marriott Pt; Hyatt Globalist
Posts: 17,831
You definitely need it traveling from the US to Mexico.
I've traveled to from US to Canada, Greece, Italy, UK and probably several other places with my son and have always carried a notarized letter from my husband, but have never been asked for it.
A friend of mine found out about the Mexico requirement when trying to leave LAX with her children to meet their father in Mexico. When she found out they wouldn't allow them to board the plane without a notarized letter, she "found" their father right there at LAX of all places and found that they had a notary at the airport. Many large airports do have notaries so all is not lost if you arrive at the airport without the letter.