Hello ALittleLostMom47
Good day.
SBB is accepting credit cards that have the 3D secure ID activated. It is most likely a Chase credit card…
You can also purchase the ticket using Deutsche Bahn, they are less picky: https://int.bahn.de/en/
There is a technical issue with the stop ‘Weil am Rhein, Dreiländerbrücke’. Neither Deutsche Bahn nor SBB can issue a ticket from that stop. If you use the ‘Weil am Rhein, Basel Grenze’ stop, it also works on the Deutsche Bahn website.

In Europe, we do use the term ‘reservation’ for purchasing a train ticket. Because a ticket and a seat reservation are two different things here. When you say, I reserve a train, it actually means, you reserve a seat on that train. And, domestic train tickets do not offer seat reservations. We simply purchase a regular train ticket for a route, and take the next train to come.
Read here:
https://community.sbb.ch/blog/23887-demystifying-seat-reservations-on-swiss-trains-a-comprehensive-guide
For children, you could purchase the Children Day Pass or the Junior Card. Both pay off also with a few train trips only (however, staying in Malters (where you probably have an AirBnB), I wondered how you will commute between the sites if not by public transport).
Kind regards,
Roland