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 Brussels. In public schools, in the third year of primary school they have to learn Flemish as it is a bilingual Region. Private schools follow other rules.
In Wallonia, it is compulsory in 5th and 6th grade although some schools start in the 3rd year. the school can organise lessons in German, English or Flemish. In Flanders, French is compulsory from the 3rd grade.
In the German speaking Region, it is compulsory to learn French or Flemish in the 3rd year.
Please check with the principal of your school to know what is the situation.