See the following CBP website link.

Air Travel
http://www.cbp.gov/xp/cgov/travel/vacation/ready_set_go/air_travel/documents_needed.xml
For air travel, it says, "Children will be required to present their own passport."

Land/sea Travel
http://www.cbp.gov/linkhandler/cgov/travel/vacation/ready_set_go/land_travel/whti_ls_faq.ctt/whti_ls_faq.pdf
For land/sea travel, children can present birth certificates/citizen papers only if "traveling in designated school or youth groups".