Originally Posted by
chtiet
Incorrect. I stand by what I wrote. The question is about
entering Mexico. Mexican law
DOES NOT require US citizens to have a passport.

Has nothing to do with entering Mexico
This being the Mexico forum, the OP's question is about entering Mexico, not entering the US, so I repeat that you
DO NOT need a passport if you're a US citizen.
True, but you will to get back into the US next year. If they live here and want to come back then they'll need one.