Train from Frankfurt to Prague

Frankfurt to Prague

Does anyone have advice for the best way to get from Frankfurt to Prague? I will be in Prague already, and my mother and teenage daughter are coming to visit. They are not experienced travelers, and my mother has NEVER been to Europe. They arrive on an international flight to Frankfurt at 8:30 am. I really do not want them to do the overnight train, as they would have to wait until 11 pm to get on it and I don't think overnight train would be good for my mother.

The flights seem to be much more expensive than any other flights I've seen from point to point in Europe. The flights later in the day are cheaper than the mid-day flights. Is it worth it - if they wait all day before the flight to Prague, is it worthwhile to do that and go into Frankfurt to see something of the city?

I have even thought of possibly renting a car and driving to get them, but I'm not sure about that, how long it takes or anything.