Every parent has the right to a monthly allowance in Belgium for his children. This allowance is called child allowance (allocations familiales/kinderbijslag).
The amount of benefits you receive will depend on various factors, you can calculate the amount you are eligible to receive here: http://vlaanderen.famifed.be/fr/calculator
You can apply for child benefits through your child benefit fund (if you don’t know which, you can ask your employer) or through FAMIFED.
For more information:
If you are not entitled to child benefit under one of the above systems, the State Service for Child Benefit for Employees (ONAFTS) grants you the guaranteed family allowance. This family allowance is subject to strict conditions concerning income and the minimum length of your stay in Belgium. Your right expires as soon as you obtain rights in one of the other systems.
For more information visit : https://www.socialsecurity.be/citizen/fr/famille/allocations-familiales-ordinaires