There are activities inside, but for private ones must generally pay. La Ligue des familles, Gezinsbond and British childbirth Trust organise playgroups in several places. Please check their website for more information.
Indoor options for children of this age also include toy libraries (ludoteques/spelotheken) and indoor “maisons ouvertes,” a list of which can be found at :
http://www.lesmaisonsvertes.be/spip.php?rubrique65.
http://www.spfb.brussels/jeunesse-petite-enfance/ludoth%C3%A8ques/ludoth%C3%A8ques-bruxelles
http://www.thevillage.be/activities/toy-libraries
Please check with your commune for more information. There are many activities proposed.
For a list of other indoor play centers in Brussels visit http://www.thevillage.be/activities/playgrounds-and-indoor-play
https://expatsinbrussels.be/en/kids/activities-workshops-courses-birthdays
https://expatsinbrussels.be/en/kids/museums-with-children-in-mind
https://expatsinbrussels.be/en/kids/amusement-parks-for-children
https://expatsinbrussels.be/en/kids/good-to-know
http://www.isb.be/uploaded/ISB_News/FA_Things_to_do_with_Children_in_Belgium_2009.pdf