There are lots of problems on this route throughout August with several ICE trains cancelled or curtailed each day. Those that are running are mostly fully booked. Look on the SNCB (Belgian railways site for more details). You could travel via Welkenraedt and Aachen but your German Interrail will not be valid via this route until you get to Aachen.  It is a lot slower though as you need to wait in Aachen for nearly one hour. It costs 24 euros for an adult ticket as far as Aachen. If you take this option you could try applying for a refund from DB but I think you will be unlikely to be successful as you didn’t have an advance reservation. I’d see what advice you get at Brussels Midi on the day or bite the bullet and give up on the ICE train. If you have young children I would go on the slow route and pay for the ticket.

It CAN be done for lots less-certainly for ju/senior, by combining tix till last stop In BE and the short hop across. Plus that RE trains AC-K run at least 2/hr-so there is no 1 hr wait.

But in essence watkins is right-even the RES does not garantee that the train will run! And these rpobs are much longer, though quite worse for now-and also apply to the sectors K-AMS in NL. In fact the today released stats of DB over july show that delays and cancellations are getting worse every month-till now less as half of all ICE trains running on time

Thanks @cdwatkins19  and @mcadv, I’ll look into other options. So just to be certain - “fully booked” means they won’t let you on and you sit on the floor? 

You can always enter a train with optional reservation, but you might not get to travel with the train. 

If the train is too crowded the train staff will, for safety reasons, ask everyone without a reservation to leave the train. If people don't get of the train they will get help from police to force people who don't have reservation to leave the train.

We don’t know whether they’ll let you get on at Brussels. Maybe someone in the community has experience of this. The major changes don’t start until next week 

What changes are you talking about if I may ask? 

Just got off the phone with Deutsche Bahn, they said exactly what @AnnaB said - in principle they let you get on and stand, but if it’s very full they might kick you off and you have to keep trying your luck.

Make sure you practice “making yourself big”. i.e. get close to the door,  protect the youngsters in front and adults act as a wall to stop pressure from behind while juniors board. Suitcases etc can be entertaining.