Le Petit Moutard (http://www.lepetitmoutard.be/), or The Little Explorer (http://www.the-little-explorer.be/?lg=3) in English, give lists of programs and activities for children in Belgium.
La ligue de famille: information on services and programs for children and families can be found at lhttps://www.laligue.be/association.
“Les Maisons Ouvertes” for young children can be found at http://www.lesmaisonsvertes.be/spip.php?rubrique65
For a small fee you can also join playgroups and interest groups through the Brussels Childbirth Trust; http://bctbelgium.org/social-life/
For information on playgroups visit http://www.expatica.com/be/family-essentials/Childcare-in-Belgium_100203.html
For more ideas of activities visit http://www.isb.be/uploaded/ISB_News/FA_Things_to_do_with_Children_in_Belgium_2009.pdf
In Wallonia/Flanders you can check with ONE/Gezinsbond/Kind & Gezin (https://www.gezinsbond.be/activiteiten), CPAS/OCMW (you will need to check for the specific commune you are living in)
An extensive list is also available under https://expatsinbrussels.be/en/kids
For older children : http://www.vgc.be/aanbod/vrije-tijd/kinderen-en-jongeren/jeugdwerk