Updated January 2014
In order to prevent international child abductions, many countries require children who are traveling without their parents to present documentation that proves that the parents authorize the child to travel. In the past, it was an official requirement of the Mexican government that any child entering or exiting the country should carry a letter of permission from their parents, or of the absent parent in the case of a child traveling with only one parent. In many cases the documentation was not asked for, but it could be requested by immigration officials.
Since January 2014, new regulations for children traveling to Mexico stipulate that foreign children who travel to Mexico as tourists or visitors for up to 180 days only need to present a valid passport, and are not required to present other documentation. However, Mexican children, including those holding dual citizenship with another country, or foreign children residing in Mexico who travel unaccompanied by either parent are required to show proof of their parents permission to travel. They must carry a letter from the parents authorizing travel into Mexico. The letter must be translated into Spanish and legalized by the Mexican embassy or consulate in the country where the document was issued. A letter is not required in the case of a child traveling with only one parent.
Note that these are the requirements of the Mexican immigration authorities. Travelers must also meet the requirements of their home country for exit and return.
Here is an example of a letter of authorization for travel:
I (parent's name), authorize my child/children, (child/children’s name) to travel to (destination) on (date of travel) aboard Airline/Flight # (flight information) with (accompanying adults), returning on (date of return).
Signed by parent or parents
Signature/Seal of Mexican embassy or consulate
The same letter in Spanish would read:
Yo (parent's name), autorizo a mi hijo/a (child's name) a viajar a (destination) el (date of travel) en la aerolinea (flight information) con (name of accompanying adult), regresando el (date of return).
Firmado por los padres
(Signature / Seal of Mexican embassy) Sello de la embajada mexicana