Some info I found on the State Departments web site, id suggest going to it and reading
Special Requirements for Children Under Age 16 Minor Children must Apply in Person
All children regardless of age (including newborns and infants) must have their own passport.
LAND AND SEA TRAVEL
The following summarizes information available on the Department of Homeland Security’s website.
JANUARY 31, 2008
U.S. and Canadian citizens will need to present either a WHTI-compliant document, or a government-issued photo ID, such as a driver’s license, plus proof of citizenship, such as a birth certificate. DHS also proposes to begin alternative procedures for U.S. and Canadian children at that time.
At a later date, to be determined, the departments will implement the full requirements of the land and sea phase of WHTI. The proposed rules require most U.S. citizens entering the United States at sea or land ports of entry to have either a U.S. passport; a U.S. passport card; a trusted traveler card such as NEXUS, FAST, or SENTRI; a valid Merchant Mariner Document (MMD) when traveling in conjunction with official maritime business; or a valid U.S. Military identification card when traveling on official orders.