Note that local, unreserved trains between Brussels and Brugge are very frequent. They are also heavily used. Some of the trains are composed of modern, "double-decker" second-class cars (which are also used on some commuter routes in the United States.) The reason I mention this is that the hallways, aisles, and staircases (both levels require steps to get to the seats) are narrow and difficult to negotiate with large bags. I have sometimes stood in the big vestibule nearer the exterior doors, when I've had a big bag.
I happen to be a heavy-packer, and I don't feel the slightest shame about it. If the traveler is willing to carry their own bag, and not injure others by thoughtlessly moving it around, they should not be scolded here. I do question the wisdom of TWO good-sized bags each. Even if you go to the gym regularly, you are likely to regret this burden on yourselves. You may wish to explore hotel laundry pricing, or to buy a few quick-dry undergarments. We used to waste a lot of time sitting in European laundromats (and damaging our clothes with 80C water.) Now, I see that our touring-time has "value", and should not be spent looking for change and sitting around.
Even on the single-level coaches that may have a luggage rack at one end, there is not enough space for everyone's bag. If First Class is available, feel free to buy it.
EDIT: Note that there is a huge difference in luggage space between premium long-distance trains and local trains. On the train from Brussels to Brugge, you can expect an open-work, lengthwise shelf over the seats that will NOT properly hold an American "carry-on". It's designed for briefcase-sized luggage. If you are lucky, you'll be off-hour and there will be some rows of seats unoccupied. Note that daily school commuting is a major part of Flanders life. The rush hour starts at 3PM!