http://can-am.gc.ca/consular_services_consulaires/border-frontiere.aspx?lang=eng#children

According to the official Canadian website above. Children age 15 and younger do not need a US Passport to travel to Canada by land or by sea. A US birth certificate is good enough when traveling with both parents, if both parents are not present and then need a consent letter from the parents authorize the child to travel with someone in particular off course like aunt uncle, hopefully not friends or stranger 🙂