Does anyone know:
Do we need to pre-book train tix from Antwerp to Amsterdam, or can we just buy at the train station? We're seeing prices from 30-90 euro one way, with a stop in Rotterdam, but we also found folks who claim we can buy when we get there. Any advice appreciated: we would need to be in Amsterdam on a specific day, thus the concern.
The Thalys high-speed train uses airline style pricing, so the sooner you book the cheaper it will be. The savings are significant. If you wait until the day of the journey you will be paying the most expensive fare meant for business travelers. http://www.thalys.com
If you don't mind changing trains, you can take IC trains on this route. For IC trains, the price is fixed and doesn't go up closer to travel, so you can buy these when you get there. But if you want faster direct service (no change of train), you want the Thalys, and as Michael said, you want to book those now.
Belgian Rail https://www.b-europe.com/Travel shows both options, so you can compare.
Thank you both for the advice!
You're on the right track with the advice above.