Babies born in Belgium can assume Belgian nationality at birth only if the parents have lived in Belgium for at least five consecutive years in the ten years prior to the birth or if one of the parents has been allowed to remain indefinitely in the country. If that is the case your child can be granted the Belgian nationality on the condition that you submit a request for citizenship before he/ she reaches the age of 12. If that is not the case, children will normally assume the nationality of their parents. If the parents have 2 different nationalities, the best is to check with the helpdesk of the CGRA.
A newborn will also receive the Belgian nationality if not granting the Belgian nationality would render the child stateless.
More information:
https://justice.belgium.be/fr
https://www.angloinfo.com/how-to/belgium/brussels
https://diplomatie.belgium.be/en/services/services_abroad/registry/registration_certificatesB