Your reply confirms Antwerp as a rich place for four days, and daytrip options anywhere. Not to sound like a Travel Agent, “Bruges is for Lovers”, and there’s not enough to do there for 11-year old boys. Antwerp has plenty of medieval sights, just not as continuous as in Bruges. It is also a more diversified and sophisticated place than Bruges. I feel very safe walking through the diverse sections of Antwerp.
The Antwerp-Brussels area is a better place to investigate the (past) long failure to form a government in Belgium, the spread of Islam in modern Europe, economic and language stresses, homelessness and immigration. You might want to take the (cheap) city bus from Antwerp to Turnhout, (and the train back) to view the prosperous suburburban development. Turnhout has a lovely cathedral, Beguinage, and Art Deco town hall, but it has always struck me as an example of a successful welfare-state family town. It has some international pharmaceutical manufacturing as well, but is a very pretty place. (I am not saying everyone needs to visit Turnhout. I’m responding to the school project.)
Edit: While Lier does not rate ahead of Gent or Brugge, train routes mean it can be done on the same full day out as Turnhout. Besides being a very pretty place, you would try the local signature pastry, Lierse vlaaikes. There is a sweet little town hall art museum, an engineering giant clock marvel, and a toy or puppet (?) museum I haven’t been to.
The university town of Leuven has a lot of 20th Century history, including the destruction and rebuilding of the magnificent University library, not just once, but twice. Also a lovely town hall, cathedral, and Beguinage.