Do children under 1 need a passport to travel internationally?

A birth certificate will work just fine. When children who are U.S. citizens under the age of 16 come into the United States by land or sea from Canada, they can use a birth certificate (original or a copy), a Consular Report of Birth Abroad or a Naturalization Certificate.

Something important to know however is that unless the child is accompanied by both parents, the child must have a notarized letter from the other parent or signed by both parents stating, "I acknowledge that my son/daughter is traveling outside the country with [the name of the adult] with my permission."