As of February 15, 2013, foreign minors (under 18 years of age) travelling alone or accompanied by a third party of legal age (grandparent, aunt/uncle, other) must present, in addition to a valid passport, a document showing the consent of both parents or those with parental authority or guardianship over the minor. If the document is issued in Canada it must be notarized, legalized by the Mexican Consulate or Embassy, and translated into Spanish.
The text of this document must contain the parents’ express authorization for the minor to travel alone or, if applicable, information on 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, as this will be a requirement for allowing the minor to leave Mexican territory. Minors travelling with at least one of their parents do not require any authorization at all.
To check up on your destination, read Foreign Affairs Canada's Travel Reports and Warnings for more information.