Are you sure that the Eurostar allows passengers to board at Brussels to Amsterdam? Generally the fast train between Brussels (Midi/Zuid) to Amsterdam Centraal is the Thalys, slower train is the International.
The new Eurostar service (one way only, there is no return train) from London to Amsterdam does call at Brussels but I had no idea they allowed new passengers to board. It must be expensive.
Check your minimum check in time for Eurostar. Usually you'd be looking at an hour, due to security and border control, but if they are allowing Brussels - Amsterdam that won't need border control, yet they must have to put everybody going to the departure happ through security else security is compromised.
Can you let us know what you have been told?