Update: The new requirement concerning travel to Mexico for minors has been postponed until January 24, 2014. More info here: Documents for Minors Traveling to Mexico
New immigration rules in Mexico came into effect on November 9 and bring a few changes for tourists. One of the main points that may affect travelers is in regards to minors (under 18) traveling without their parents. According to the website of the Secretaría de Relaciones Exteriores, minors traveling alone or accompanied by an adult third party must present, in addition to a valid passport, a document showing the consent of both parents or legal guardians of the minor. If the document is issued outside of Mexico it must be translated into Spanish and notarized and legalized by the Mexican Consulate or Embassy. An unofficial translation is sufficient. The text of the document must state the parents' express authorization for the minor to travel alone or, if applicable, information about the adult who is authorized to be responsible for the minor during the trip. The document must state the purpose and length of the trip, and specify the dates of entry and departure. This document is a requirement for allowing the minor to depart from Mexico.
According to this statement, a child traveling with only one parent does not need an authorization letter. I've always recommended that a child traveling with only one parent carry an authorization letter from the other parent (or a document proving sole custody). I still think this is a good idea to be on the safe side, and although Mexico is not making it a requirement, it may be requested on the return trip.
Another important change in the new immigration laws is that someone who is in Mexico as a tourist may not apply for residency. So if you're planning to come to Mexico to work or stay long term, you should apply for an FM2 visa at the Mexican embassy in your home country before entering Mexico. In the past it was possible to come as a tourist and stay for six months and then switch to an FM2. That won't be possible anymore.
For more about entry requirements and travel documents, check out the following links: