If seats are available for pass users you can book also via b-europe via this link:

https://www.b-europe.com/EN/Booking/Pass#TravelWish 35€/seat + 4€ booking fee/booking

or via eurail.com where it costs 35€/seat +2€ booking fee/seat

If no seats are available for the direct services, you can use Eurostar services to Brussel (30€/seat) and change there to an hourly Intercity train to Amsterdam (reservation not needed and not possible)

Okay I see that I can book London To Brussels on Oct 16th 

So I could confirm and then as you noted take a train From Brussels - same terminal that we come in from from LONDON and hop on an intercity train to Amsterdam Central.

Yes. The IC Brussels - Amsterdam run hourly and require no reservations.

So if Eurostar arrives at 12:05 pm we are okay to get on this IC BRUSSELS to Amsterdam Central at 12:28 pm??? enough time? 

Yes, you can go to Brussel by train (Eurostar has security checks like at airports, be some time before 45-60 or more minutes in London St. Pancrats station. 

In Brussel you get at the train station Bruxelles-Midi/Brussel-Zuid. Thats in English the south station. You only have to change trains from one platform to the other. IC Trains to Amsterdam start every hour at xx:45. 

If you have time you could use half a day in Brussel to visit the city center. You can travel by train (local and IC trains) to the station Bruxelles-central/Brussel-Centraal and get directly in the city center. There stops also the IC to Amsterdam. 

If you want to go by high speed train there are Thalys high speed trains, they need a compulsory reservation (the time difference between IC and Thalys is not big).

I can't see an IC train at 12:28 pm. I see it at 12:45. Note in Europe we use military time with 24h.

If you don't get this one the next is one hour later.