Cheapest way to travel back from Zaventum airport to Woluwe-Saint-Pierre

On a working day, what is the cheapest way to travel back from Zaventum airport to Woluwe-Saint-Pierre(Area Chant d'Oiseau)? The only catch is, a mom plus two teenage boys and one child will be in the group, with 4 checkin bags and 4 cabin bags.

-Is the only option to book a big taxi?
-If yes, how/where to book one?
-How much do they(big taxis) charge, double the fare of normal taxis?

Thanks in advance