Today Mexico published in the DOF new rules for minors leaving Mexico who are unaccompanied or with an adult who is not their parent or guardian. The publication will take effect in 30 days and mentions a possible official permission form which would need to be notarized, apostilled or legalized and translated (if not in Spanish), form to be published within 30 days. Curiously the new law doesn’t mention visitantes perhaps exempting tourists from this requirement as it only lists Mexicans and residente temporales and permanentes who need to have the permission letter, also fines of up to 10,000 minimum salaries to transport companies who don’t check that the minors have the letter.
Immigration’s web site lists the procedures here: http://www.inm.gob.mx/index.php/page/autorizacion_menores