A Mexico Visitor's Permit, called an FM-T, is needed if you plan to visit Mexico for longer than 72 hours or visit outside the "free zone" (defined as the an area between 20 and 30 kilometers from the border with the US). The FM-T is only available to holders of US and Canadian passports, in place of a visa. It is available at Mexican consulates, Mexican border crossing points, and Mexican tourism offices.
You will need a Passport or Passport ID card for each individual traveling into the interior of Mexico in order to obtain an FM-T. There is a fee associated with obtaining the FM-T.