As the grandparents may be aware, the Arrivals hall in YUL is a bit of a zoo. Checked luggage can take + 20 min. I walk out of the secure area and make a bee line for the up escalator for my cnx. Note that is how you "go back through security". My advice would be to pick a nicer place for the rendezvous. Grandparents may walk to park @ the Sheraton and take the shuttle over? Porter's last departure is at 21:15 i.e. there is little risk of a misconnect!