Schools will often provide some sports activities for free or at a moderate price. Outside school there are also many private or public sport clubs, but these need to be paid for.
There are various youth clubs for children this age, the most common one is the Scouts
The main federations include:
The Scouts en Gidsen Vlaanderen
Les Scouts - Fédération des Scouts
Baden-Powell de Belgique
More information on : http://www.scouts.be/
In Flanders it is also common for children this age to join the youth club Chiro. The Wallonian equivalent is called Patro.
For an extensive list of activities for children this age you can visit https://expatsinbrussels.be/en/kids/activities-workshops-courses-birthdays