No flights to CRL? I'd go to London City or Heathrow to get that, even though it's not BRU. I can't endorse yet another border crossing in Covid time, either. I mean, who would fly to Northwest Arkansas to get to Kansas City, MO? (I've been both those places!) Are you expecting something from the Belgian train ride that it may not deliver? It's a long, dreary trip with at least one change. At least the Eurostar is plush and seamless. (If some risk of a seat change nearer Bruges.)
Have you or your family ever been to Europe before? I just wonder, respectfully, if you are applying a "Jump in the SUV cocoon" mindset, to significant cross-border travel? (Disclosure: I grew up in a city, and neither of my parents ever owned a car their entire lives.) Lining up for Eurostar customs is no fun, but it's nothing like the "All Other Passports" queue at an airport! Have you observed what happens to visitors to the U.S.A.?
Edit: Are you staying in East Anglia? There are lot of lovely destinations nearby, especially if you have read the Swallows and Amazons books. Norfolk Broads, half-timbered town remnants, Folk museums, a railway museum, Ipswitch, universities, historic mansions with no crowding, and more.
Has the family been to Amsterdam? It's not as "beautiful" as Bruges, but it is mighty pretty, and it has far more things to do. It can't be seen in less than a week. And it has just as many train daytrips as does Flanders. Like Bruges, it is not well-connected to Great Britain. But I believe Eurostar now serves Amsterdam, due to demand.
BTW, the Eurostar London terminal is a quite nice adaptive re-use of a train shed railroad station. Again, it's not as mechanical as an open train assembly and storage yard.