To enter the US, your passport must be valid for the length of your planned stay. If you're travelling through another country on your way to or from the US, check the entry requirements for that country. Many countries will only allow entry if you have at least 6 months validity remaining on your passport.
If traveling by air, an unexpired Passport is required for everyone, including infants.
Your child may not need a passport to travel to the US by land or sea. Children 15 and under don't need a passport to enter the United States or return to Canada by land or sea. It should be issued by the vital statistics department from the province or territory where they were born.
If one parent is not available, the non-applying parent must complete the form DS-3053 . This form must be notarized, and the identification that was presented to the notary must be copied, back and front, and included with the form. We require the original of the DS-3053 to adjudicate the passport application.
When U.S. citizen children under the age of 16 arrive by land or sea from Canada or Mexico they may present an original or copy of their birth certificate, a Consular Report of Birth Abroad, or a Naturalization Certificate.
Legally or developmentally? Legally, it depends on your jurisdiction, however, by and large, children need the permission/approval of both parents to leave the general area of their home... To travel internationally, they need a passport and the consent of both parents, unless one parent has sole legal custody.
Typically, minors under the age of 18 do not have to present identification for domestic U.S. travel. Airlines will accept identification from the responsible adult on behalf of the minor(s). For international travel, minors under the age of 18 must present the same travel documents as the adult.
Submit a Statement of Consent: Form DS-3053 or a notarized statement from both parents or guardians giving that person (example: grandparent) permission to apply for the child. Include a photocopy of the photo IDs for both parents or guardians.
While the US does not legally require a parental consent form, US Customs and Border Protection strongly recommends one. Here's the advice from their Web site, bolding mine: