http://www.usembassy-mexico.gov/eng/eacs_sheet.html
Unaccompanied Minors Departing Mexico
In order to combat international child abduction or the exploitation of minors, Article 215 of Mexicos Ley General de Población requires that minor non-Mexican children leaving Mexico must be accompanied by both parents or guardians or be prepared to present written authorization to travel from the absent parent or parents.
This document must include:
the name(s) of the authorizing parent(s),
the name of the child,
the name(s) of the adult(s) accompanying the child, and
the notarized signature(s) of the absent parent(s)
The child must be carrying the original letter not a faxed or scanned copy as well as proof of the parent/child relationship (usually a birth certificate) and an original custody decree, if applicable.
If there is only one custodial parent, the accompanying adult should be prepared to present documentation to that effect (e.g, a court order granting sole custody to one parent, a death certificate for the absent parent, etc.).