It will depend on the Region where the child goes to school. It is a mandatory part of the school’s curriculum to learn another language in Belgium since primary school. Private schools follow their own rules.
In Wallonia, it is compulsory to learn Dutch or English. In Brussels, as a bilingual Region it is compulsory to learn Flemish. In Flanders, French is compulsory.
In the German speaking Region, it is French or Flemish.
Please check with the principal of your school to know what is the situation.