I did not read all of Ben's links but returning to US is probably hardest part. From DHS:

Children:  U.S. citizen children ages 15 and under arriving by land or sea from a contiguous territory may present an original or copy of his or her birth certificate (issued by the Vital Records Department in the state where he or she was born), a Consular Report of Birth Abroad, or a Naturalization Certificate.  If the child is a newborn and the actual birth certificate has not arrived from the Vital Records Department, we will accept a Hospital issued birth certificate.

Note that for travel by air, passport is required.

After studying this for my kids, I decided the passport made sense in the long run, making it easy to renew for rest of their lives. One of my sons used it a few years later as an exchange student.

Great fun ahead!