Please check with your commune to know which schools are available and their languages. Some schools offer languages courses such as Russian, Arabic etc during Wednesdays and weekends.
There are language schools for children that offer a variety of different languages, but usually have to be paid for. Please check with your commune for more information.
For more information: http://www.centreislamique.be/node/258
http://www.culture-arabe.irisnet.be/Cours%20pour%20enfants.htm