Seat reservations on this route are optional, despite what Eurail may say. You can book reservations for free, however, at a ticket office in the UK or via myaccount.gwr.com. You first have to create an account. (It weirdly doesn't accept spaces in the surname.). Then sign in, and click Bookings at the top. Then look for the “Make a seat/bike reservation“ box. It will look as if you need to select a fare, select any, it doesn't matter (do check 1st or standard=2nd class), continue and make the reservation. It says you won't be charged, and indeed you get the free stand-alone seat reservation without needing to enter any payment details. Even though it's GWR, this works for any route where seat reservations are available.

Ok.  So I make the seat reservations on gwr and then we use the Eurail pass to purchase at the train station?  Or do we just board the train?  
 

Your Eurail Pass is your train ticket. You only need on some trains a reservation. Most trains in United Kingdom don’t need a reservation and they are only optional. 

So just hop on. 

* Don’t forget to add your trip to your Eurail Pass via App or paper pass. 

Nice.  I shouldn’t have a problem am getting first class seats for the 5 of us?  

That should be no problem.

You can REServe at ANY counter anywhere in UK and also ahead-so take that chance. I´ve BTW done a seat-REServ for myself 2 days ago at Edin and it was done in just a few secs and free too. But you run the risk as you have a ´party´ that not all 5 can sit closely together-as seems sometimes an utmost frightening prospect. IN SCOTld masks are still being worn in transit

Thanks for the information.  We live in the US and won’t be in UK until May 28th.  Train to London on May 31st.  Maybe I’ll reserve the seats online to make sure we can sit together.  Also have a special needs child in a wheelchair, so want to make sure I get seats together in first class.