Rules for Car seats for children

Assuming you can read French, here is a pretty good summary:

https://www.moniteurautomobile.be/conseils-auto/code-de-la-route/les-sie....

The important point is the law says that if your child is less than 1.35m you need an appropriate car seat.

Just be careful though if you are a regular traveller to other EU countries as there is not an EU standard and quite a few countries have different standards for age / weight / height. For example, if you travel regularly to Germany, the threshold is 1.5m.

The basic rule is indeed that children under 1.35m tall need to be in a suitable car seat. They can be in the front at any way, provided they're in an appropriate seat. There are also some exceptions to the car seat rule, notably if you're doing occasional carpooling, or travelling in taxis, buses etc.

There's a good overview here: https://www.thevillage.be/parenting/car-seat-rules-in-belgium/

* Kids can be seated in the front of a car at any age, IF they're in an appropriate car seat, and with the airbag deactivated of course if it's a rear-facing seat.