Depends hugely on the crossing and employees. You technically need a passport for every individual crossing, and can be denied the ability to transport a child without express parental permission (we bring a notarized letter when travelling with my much younger sister). However, the enforcement of these guidelines is very spotty in my experience. It has ranged from very rigid to knowing individuals who cross regularly without a passport. Smaller more rural crossings often seen more laid back. I think this is partially due to the fact that passport requirements for US/Canada land crossings is a relatively new phenomenon, and citizens and workers alike are not all thrilled about the change. I can't speculate about this particular crossing, as my times travelling to the states have been via Ontario and New Brunswick, but wanted to share based on my experience.
Perhaps someone has crossed there specifically
Edited to add: I have also never had my vehicle peeked into/searched. An elderly couple may be less likely to raise suspicions and could plausibly hide a child for the duration of a crossing. I am not up to date on this case or thread, despite tons of local media attention, so I am not sure of the specs of their vehicle or wheher we know if the grandparents had access to their own passports.
:seeya:
