There's no reason not to make advanced seat selections for your family.
It's free and ensures that you sit together without having to relocate others or add to the an agent's workload at the airport:
If you are travelling with children under the age of 12, you will receive complimentary seat assignment ensuring children are seated adjacent to an adult/guardian travelling with them. You should always indicate in your booking that you are travelling with children.
You may contact Air Canada Reservations directly to be seated, or go to the ‘Manage My Bookings’ tab to review your reservation 36 hours after booking to validate your seat assignment.
I don't know what families "tend" to do, but you should definitely make advanced seat reservations.