My relatives never even much cared about making out such letters, and now we skip the letters all the time for Canada-US cross-border traffic (in either direction).

If CBSA or CBP want to make an issue over not having an easily doctored/misleading letter when the passengers' counter-offer is a live video conference call or a video recording with the legal custodial parent(s)/guardian(s) not present, then the migration control types were looking to make a stink for no good reason from the start and additional paperwork the passengers present won't necessarily do any good (and could even perhaps make things slower/worse).

It's rather amusing when people think a letter will spare them hassle, but then they soon thereafter get sent to secondary and end up worse off than other members in the family cleared by the same officers under much the same circumstances but without having any such letter to show -- especially when all the extra hassle arises because of the notarized letters' feeding more suspicions in the heads of the authorities who have already jumped to a suspect conclusion about the individual(s) which CBP/CBSA presumed to be suspicious.

"Can I see a letter from ___ parent/parents authorizing this travel for the children?"

"Don't have such a thing since I travel all the time with the kids by myself, but you're welcome to video call _____. Here's _____ live from the kid's phone."