In crèches, they are usually enrolled on a regular basis. There are a number of other facilities (haute garderie, gardiennes etc) that can accept children on an ad hoc basis or part time. For example, in la maison des enfants (the Children's House in Etterbeek you can bring your child for a few hours on certain days. http://www.etterbeek.be/nos-services/enfance/the-house-of-children
Please check with your commune or ONE/ Gezinsbond to know which options are offered and to know which communal creches there are.
You can also call a babysitter. La Ligue des familles puts parents and babysitter in touch (https://www.laligue.be/Public/babysitting .
At the Flemish level, Gezinsbond offers the same service https://www.kinderoppasdienst.be
The app Bsit also puts in contact parents and baby sitters: https://bsit.com/en/parent
For any of these options usually you will need to provide proof that your child has been vaccinated.
For more information:
https://www.expatica.com/be/family-essentials/Childcare-in-Belgium_100203.html http://www.thevillage.be/parenting/daycare#vaccinations