According the the Canada Border Services Agency website, If you are a US citizen you don't need a passport to enter but you need proof of citizenship, such as a birth certificate, certificate of naturalization or citizenship and also a photo ID. If you are a US citizen but a permanent resident of Canada have your Permanent Resident Card handy. Having stated this however the US State Department Website states that returning US citizens need a Passport Book or Passbook Card to get back in!
Parents who share custody of their children should carry copies of the legal custody documents. It is also recommended that they have a letter of authorization from the other custodial parent to take the child on a trip out of the country. Such a letter will confirm that the child is not being abducted or taken against his/her will. The parents’ full name, address and telephone number should be Included in the letter of authorization.
When traveling with a group of vehicles, parents or guardians should arrive at the border in the same vehicle as the children.
Adults who are not parents or guardians should have written permission from the parents or guardians to supervise the children. The permission letter should include addresses and telephone numbers where the parents or guardian can be reached.
CBSA officers watch for missing children, and may ask detailed questions about the children who are traveling with you.
The information below is from the US Department of State Website/Mexico
ENTRY / EXIT REQUIREMENTS: For the latest entry requirements, visit the Embassy of Mexico’s website (good luck finding useful information there) or contact the Embassy of Mexico at 1911 Pennsylvania Avenue NW, Washington, DC 20006, telephone (202) 736-1600, or any Mexican consulate in the United States for the most current information.
All Americans traveling by air outside of the United States are required to present a passport or other valid travel document to re-enter the United States. This requirement was extended to sea travel (except closed-loop cruises), including ferry service, on June 1, 2009. Starting June 1, 2009, all travelers entering the U.S. by land, sea or air were required to present a Western Hemisphere Travel Initiative (WHTI) compliant document such as a passport or a passport card. While passport cards and enhanced driver’s license are sufficient for re-entry into the United States, they may not be accepted by the particular country you plan to visit; please be sure to check with your cruise line and countries of destination for any foreign entry requirements. U.S. legal permanent residents in possession of their I-551 Permanent Resident card may board flights to the United States from Mexico.
Applications for the U.S. passport card are now being accepted and have been in full production since July 2008. The card may not be used to travel by air and is available only to U.S. citizens. Further information on the Passport Card and can be found on our web site. We strongly encourage all American citizen travelers to apply for a U.S. passport well in advance of anticipated travel. American citizens can visit Bureau of Consular Affairs website or call 1-877-4USA-PPT (1-877-487-2778) for information on how to apply for their passports.
The U.S. Department of State is unaware of any HIV/AIDS entry restrictions for visitors to or foreign residents of Mexico.
Minors: Mexican law requires that any non-Mexican citizen under the age of 18 departing Mexico must carry notarized written permission from any parent or guardian not traveling with the child to or from Mexico. This permission must include the name of the parent, the name of the child, the name of anyone traveling with the child, and the notarized signature(s) of the absent parent(s). The State Department recommends that the permission should include travel dates, destinations, airlines and a brief summary of the circumstances surrounding the travel. The child must be carrying the original letter – not a facsimile or scanned copy – as well as proof of the parent/child relationship (usually a birth certificate or court document) – and an original custody decree, if applicable. Travelers should contact the Mexican Embassy or the nearest Mexican consulate for current information.
Tourist Travel: U.S. citizens do not require a visa or a tourist card for tourist stays of 72 hours or less within "the border zone," defined as an area between 20 to 30 kilometers of the border with the U.S., depending on the location. U.S. citizens traveling as tourists beyond the border zone or entering Mexico by air must pay a fee to obtain a tourist card, also known as an FM-T, available from Mexican consulates, Mexican border crossing points, Mexican tourism offices, airports within the border zone and most airlines serving Mexico. The fee for the tourist card is generally included in the price of a plane ticket for travelers arriving by air. Please note that travelers not in possession of their FM-T card at the point of exit from Mexico may face a fine from Mexican Immigration (INM).
Business Travel: Upon arrival in Mexico, business travelers must complete and submit a form (Form FM-N) authorizing the conduct of business, but not employment, for a 30-day period. Travelers entering Mexico for purposes other than tourism or business or for stays of longer than 180 days require a visa and must carry a valid U.S. passport. U.S. citizens planning to work or live in Mexico should apply for the appropriate Mexican visa at the Mexican Embassy in Washington, DC, or at the nearest Mexican consulate in the United States.
Vehicle Permits: Tourists wishing to travel beyond the border zone with their vehicle must obtain a temporary import permit or risk having their vehicle confiscated by Mexican customs officials. At present the only exceptions to the requirement are for travel in the Baja Peninsula and in the state of Sonora, and only for vehicles entering through the Nogales port of entry. To acquire a permit, one must submit evidence of citizenship, title for the vehicle, a vehicle registration certificate, a driver's license, and a processing fee to either a Banjercito (Mexican Army Bank) branch located at a Mexican Customs (Aduanas) office at the port of entry, or at one of the Mexican consulates located in the U.S. Mexican law also requires the posting of a bond at a Banjercito office to guarantee the export of the car from Mexico within a time period determined at the time of the application. For this purpose, American Express, Visa or MasterCard credit card holders will be asked to provide credit card information; others will need to make a cash deposit of between $200 and $400, depending on the make/model/year of the vehicle. In order to recover this bond or avoid credit card charges, travelers must go to any Mexican Customs office immediately prior to departing Mexico. Regardless of any official or unofficial advice to the contrary, vehicle permits cannot be obtained at checkpoints in the interior of Mexico.
Travelers should avoid individuals who wait outside vehicle permit offices and offer to obtain the permits without waiting in line, even if they appear to be government officials. There have been reports of fraudulent or counterfeit permits being issued adjacent to the vehicle import permit office in Nuevo Laredo and other border areas. If the proper permit is not obtained before entering Mexico and cannot be obtained at the Banjercito branch at the port of entry, do not proceed to the interior. Travelers without the proper permit may be incarcerated, fined and/or have their vehicle seized at immigration/customs checkpoints. For further information, contact Mexican Customs about appropriate vehicle permits.
DUAL NATIONALITY: Mexican law recognizes dual nationality for Mexicans by birth, meaning those born in Mexico or born abroad to Mexican parents. U.S. citizens who are also Mexican nationals are considered by local authorities to be Mexican. Dual-nationality status could hamper U.S. Government efforts to provide consular protection. Dual nationals are subject to compulsory military service in Mexico; in addition, dual national males must register for the U.S. Selective Service upon turning 18. For more information, visit the U.S. Selective Service website. Travelers possessing both U.S. and Mexican nationalities must carry with them proof of citizenship of both countries. Under Mexican law, dual nationals entering or departing Mexico must identify themselves as Mexican.