It does not matter if the parents are married, the child only needs to be listed on the 25/26 form of one of the parents. If you cannot prove that the child is yours, you will need to contact the Helpdesk of the Office of the Commissioner General for Refugees and Stateless Persons in order to obtain a birth certificate if you can’t obtain it from your national government.
Contact details of the Helpdesk:
cgra-cgvs.documents@ibz.fgov.be