U.S. citizen children under age 16 arriving by land from Canada or Mexico may present an original or certified copy of his or her birth certificate, Consular Report of Birth Abroad, ... If you travel beyond the 22 miles/35 km "free zone" after crossing a land border into Mexico you will need to pay a fee of around US $22. read more
You need a U.S. passport to cross the border into Mexico in most cases. You can use a passport card if you're driving, but need a passport book for air travel. read more