Thanks canadiancow, wigg, and tcuhrc for your help!

I wrote in to the flight pass desk and this was their response:

"... Children can be added as travelers to multi-passenger Flight Passes in the same way as any other traveler. If travelling as an unaccompanied minor, Flight credits can be booked online for direct nonstop flights only. You will need to contact Air Canada Reservations to request the Unaccompanied Minor service. Flight Passes can only be purchased for children between the ages of 8 and 11 years of age. ..."

So it looks like there is likely no solve for me + family with AC, and so I will probably do a Porter pass instead as recommended above.

I also ended up buying a small AC flight pass for myself today, although I have been unable to log into the app or use the pass via the website ~4h later. So booked a last minute flight instead.

Also, for posterity, the website explicitly says "Important: Your flight credits will be available for booking 24 hours after purchase, once purchase procedures have been finalized." So that's good to know - I saw previous reports of faster bookings online, but maybe those were old posts or it's because I'm a first time flight pass buyer.

Anyway, thanks again for your help!