Letter for permission to take kids out of the EU on holidays

Yes can get this from your commune. It's called a "Autorisation parentale" in French. No idea in Dutch - sorry.

Also, you should know that there is absolutely no legal requirement to have one. There are in fact, no formal rules for travelling with children (unless there was something specifically written in your divorce agreement relating the the custody of your children).

However, if you have one of these docs, it may make things easier if there was ever any question from travel / legal authorities.

If you're in Brussels (but all communes will have an equivalent)
https://www.bruxelles.be/autorisation-parentale-de-voyager-letranger

The letter doesn't do much, normally the airline will ask for one if you have put an alert/ no fly order on the child's passport