Is your Son also interested in European travel. If so, he should come to our monthly travel meeting at Panera Bread at Aspen Grove, a little south of Lakewood.
So, at 28,-€, purchased at a Bahn automat just before travel, Regensburg to Praque for 2 would cost 56,-€. The ticket will be for a specific train, so make sure you can catch that train (day and time). Of course, if you purchase your ticket on the day of travel, or the day before, that's probably not going to be a problem.
There are other, less expensive options, most of which would require advance purchase (same price as at the time) from the Czech Rail Ticket Shop (CD.cz/en). These tickets are non-returnable and train specific.
An option that might avoid the advance purchase from CD.cz would be to purchase a Bayern-Böhmen-Ticket (Böhmen is German for Bohemian) for 37,60€ (29€ + 8.60€ for 2nd person) from a Bahn automat just before travel. The Bayern-Böhmen-Ticket is valid to Pilsen. From Plsen to Prague the Czech Rail fare for two to Prague is 310 CZK or 12,74€ today (50,34€, total, for Regensburg to Prague for 2).
Once the train is in Czechia, it should be a Czech train and the conductor should sell you tickets on to Prague for the Czech price. Ask him before you get to Pilsen. If not, get off the train in Pilsen and buy a ticket for 12,74 for the next ALEX train in 2 hours. Once you get off the train, I doubt that there is enough time to purchase your tickets and get back on the same train. Take the time to check (Czech) out Pilsen.
In 2012, I was coming up from Klotovy and was supposed to connect to the ALEX train in Pilsen. My train was late getting to Pilsen and I watched the ALEX train departing as we arrived. I took the next train, an hour later, to Prague. It was a Czech train (certainly from the Communist era, maybe even pre-war, and not very well maintained. I would advise waiting the extra hour for the ALEX train to Prague.