It looks like you need 3 seats on certain flights and have enough points to get 2 of the 3 as rewards. If that's the case, then I'd make a reward redemption for one parent and one child first, then buy a ticket for the other parent as a regular revenue ticket on the exact same itinerary. During seat selection, simply choose 3 adjacent seats.

You should also call Air Canada and ask them to "link the PNRs" so there's an indication in their systems that you're all flying together despite being on two separate bookings.