Belgium and France

See https://www.ricksteves.com/europe/belgium for a good summary of sightseeing in those Belgian cities.

A couple of considerations when deciding which direction to go first:
* Thalys train tickets to/from Paris can be much cheaper if purchased ahead (available now), but will be useless if any flight delay causes you to miss a train. That could be a reason to visit Belgium first and take those longer train rides when you have one less factor out of your control.
* If you go to Paris first, and Belgium later, then that puts you closer to your departure airport and might possibly mean fewer hotel moves, e.g., not coming back to Belgium for a one-night stay.

I assume that it's too late for you to move any part of the flight plan to a Paris airport.

Brussels bag storage info is at https://www.brusselsairport.be/en/passengers/at-the-airport/services-at-the-airport/baggage-services.

Have you considered Amsterdam or other towns in the Netherlands? That would probably be more interesting for the boys than Bruges and Ghent.

I can consider Amsterdam or even another town in France or other nearby countries. Any suggestions are appreciated

Seems like I have to choose between 3 days in Amsterdam or 3 days in Brussels (day trips to gent and Bruges on 2 of those days, third day in Brussels) ? what do you recommend? Are there things for kids to do in Amsterdam? Going to Amsterdam and then to Paris will kill half a day

If everyone rides bikes, you can rent them by the day (avoid the ones by the train station, they are more expensive) and get around Amsterdam like locals. I enjoyed the Maritime Museum and it looked like lots of fun for kids. You can wander around an old ship, and there's a multi-media presentation that's good. Take a day trip to The Hague and go to Madurodam. And it's just so different with the tall, narrow buildings leaning forward, the canals and bridges - especially after dark. The child in me says A'dam is much more fun and interesting than Belgian cities.

You are taking a risk by asking for internet information instead of looking at a comprehensive guidebook in a public library, say. It's very important to use your own knowlege of your children's interests, stamina, and patience, to leaven the suggestions you receive here. You are also attempting (I think) to encapsulate all of Western Europe in a short break in only one region. You have to plan carefully to avoid disappointment.

Amsterdam is a much more family-friendly attraction than Brussels, although most museums (in either place) are not free admission. If you want to avoid the travel time to Amsterdam, you could consider Antwerp, in Belgium, which I find more festive and varied than Brussels. (Need I write, that is an OPINION, not a FACT!) There is a nice outdoor "historic building" museum with its own rail stop in Bokrijk, Belgium. It's very popular with families and children. There is a modest "Amusement Park" of some kind right there, but I haven't been inside. The town of Leuven, which is close to BRU, is much smaller than Antwerp, family-friendly, and is kind of a Belgium in miniature, with even a small Beguinage that will give a taste of local history without wearing out little legs.