You don't need a passport if you are coming back from Tijuana, I asked the immigration people and they said it's fine with the ID and Birth Certificate. Actually if you go walking there are 2 lines one for passport and the other one is for people that don't carry passport.
Yes.
Yes, the three of you will need a passport.
Yes you do.
If you are already in Mexico, sure you can.
Yes, passports are required on flights between most countries these days.
U.S. citizens do not need a passport to enter Mexico by land or sea. The Mexican officials at the border do not ask for any identification from you. On your return home, the U.S. officials would like you to carry a current passport but they can't obligate you to do so. Every day hundreds of people cross from Tijuana to San Diego with only a driver's license. See related links section.
No. but to travel out or into the country they do.
Yes, but you will not be able to reenter the US without a valid passport. (source below)
No. Going beyond border cities such as Tijuana, Mexicali or Juarez (the later is highly unadvised to visit), you will need a passport.
A passport & a lot of protection ! People are getting mugged down there left & right. Stay out of Tijuana until they get better protection for tourists !
If you have an ID as a Mexican national or a passport as an international visitor, you can travel between both cities without an issue. Note without such ID you won't be allowed to board the plane.