I would by a refundable ticket for the 6:31 Thalys train. If you don’t make it, get your money back at the Schiphol train station and buy a ticket for the 7:09 IC train. (No need for advance reservations - and you can ask for a ticket all the way to BRU airport I think.)

Following Aviatrix’s advice, if you have to take the 7:09, change trains at Brussels Central to get to BRU at 10:08 (which should be early enough for your check-in).