Originally Posted by
Often1
Note the specific requirement for the US entry. As a practical matter, US citizens can't be denied entry, but that can be a lengthy and unpleasant experience. Now, might be a good time to get the kids passports.
Children: U.S. citizen children ages 15 and under arriving by land or sea from a contiguous territory (Canada or Mexico) may present an original or copy of his or her birth certificate (issued by the Vital Records Department in the state where he or she was born), a Consular Report of Birth Abroad, or a Naturalization Certificate. If the child is a newborn and the actual birth certificate has not arrived from the Vital Records Department, we will accept a Hospital issued birth certificate.
The document from above will also suffice for the Alaska - Canada transit as the birth certificate is proof of citizenship.
In my experience of crossing by land between Canada and the US with young children who are US citizens, my extended family have had no lengthy and unpleasant experiences at US landports of entry as a result of showing birth certificates/US CRBAs (not even always originals) to US CBP for young US citizen children instead of passports for such children. [And my family includes various people of non-European ethnic backgrounds too, in case that may matter.] This isn't the experience of someone doing this only once or twice in a lifetime -- it's a body of experience of doing this even for commute purposes.