You must present a valid U.S. passport or passport card (Six months minimum validity required for entry) in order to enter Mexico..
All U.S. citizens traveling outside of the United States by land or sea (except closed-loop cruises) are required to present a passport or a passport card, to return to the United States.
Travelers with passports that are found to be washed, mutilated, or damaged may be refused entry to Mexico. Please be sure to apply for a U.S. passport well in advance of anticipated travel.

Travel with Minors: If you plan to travel to Mexico with a minor who is not your own child or for whom you do not have sole, full legal custody, you may be required to present a notarized affidavit of consent from the minor’s parent(s) stating they are aware their child is traveling with you. There is no specific form for this document, but it should include beginning and ending dates of travel, Child/children’s full legal name and date of birth, relationship of child/children to you, full names and relationships of adult(s) traveling with the child/children. Your full legal name(s)emergency contact number and photocopies of your state-issued IDs.