Paying a housekeeper/nanny- agency?

Hi- we're looking for a live-out housekeeper that will also help with childcare (after school etc). Are there any agencies in Brussels that you can recommend? I'd prefer to go through an agency and have everything above board.

Am I correct in assuming that most of the help available on this "small ads" section expect to be paid in "black" (i.e. under the table)? Is this the most common method of hiring help in this country? (We already have a cleaner and pay her with titre service, but I understand that you cannot pay for childcare with titre service.)