I crossed back into the US from Sonoyta on Feb 8, 2010, and I had some time to make small talk with an Agent while they x-rayed my boat, so I asked them about the Passport
question. Here's what she said:
Adults must prove their citizenship-
1) Birth certificate alone will not do it because it is not a picture ID.
2) Driver's license alone will not work because it only proves RESIDENCE, not citizenship.
2) Birth Certificate plus Driver's license WILL work for adults born in the US. The DL serves as the picture ID, and the birth cert proves your citizenship. Obviously, your name on your DL and Birth cert has to match.- NOTE that this method ONLY works for US citizens BORN in the US- Naturalized citizens born outside the US can not use this method.
3) Passports, of course- work just fine and require no other source of ID.
Children under 18 who do not have a passport can only cross in the company of a parent or legal Guardian:
1) The parent or guardian has to prove their own citizenship using options 2 or 3 above.
2) The parent can prove the childs citizenship by producing the childs' birth certificate, showing the parent's name AS IT IS WRITTEN on the parent's picture ID (Driver's License or passport)- Legal guardian's have to provide documented evidence that they are the child's legal caretaker. Again- the names have to match the adult's picture ID
So, the only difference between children and adults is that children do not have to provide picture ID- providing that they are travelling with their parent/guardian.
Now, all that being said, my wife and I are just going to go ahead and get passports for our kids and be done with it...