If you are flying you need a passport. If you are driving over the border you can get a passport card, which is cheaper. If you are only going over the border for a day then I am not sure what you need.
does a citizen of Mexico need a passport to go from the us to Mexico and back? He is legal to be in us but not a us citizen.
Yes, children need a passport to travel to Mexico.
Sure, if they intend to go on vacation and then return to Mexico. However, to enter US territory, they need a passport and a Visa, which sometimes is difficult to obtain.
U.S. citizens do not need a passport to enter Mexico by land or sea and they do not need one to return home. Mexican citizens need a "WHTI-compliant document" to enter the U.S. and they may enter Mexico with nothing. The Mexican officials at the border do not ask for any identification from anyone. On your return home, the U.S. officials would like you to carry a current passport but they can't obligate U.S. citizens to do so. Every day hundreds of people cross from Tijuana to San Diego with only a driver's license. The passport question is controversial and confusing. Because of that, a group of people at the San Diego-Tijuana border put together a FAQ on the subject, see related links.
No. But they will need appropriate identification if you wish to fly.
Yes, as a tourist, you need to obtain an FMM (Forma Migratoria Multiple) to enter Mexico.
No, you don't.
Get permission.
No, just an acceptable proof of identification
To buy the plane ticket to Mexico, no... there is no ID required. But to USE that ticket and GO to Mexico then yes, you will need a passport and that will be your ID.
Your plane ticket and a passport.
On an airplane.