The supplementary family allowance for disabled children is in addition to the regular family allowance for the parents of a child aged between 0 and 21 who is disabled or afflicted. You can benefit from this additional family allowance up to the month in which your child turns 21.
http://www.kids.partena.be/content/default.asp?PageID=20
https://www.angloinfo.com/how-to/belgium/healthcare/people-with-disabilities/disability-benefits