You all have been so helpful with these train plans. Thank you. We are going to pick Kaufering as you suggested It leaves Paris at 10:55 so my parents don't have to hurry for an early start and gets into Kaufering at 17:18 so I should have enough time to get to the car rental place. And I will call for a taxi in advance. Thanks for the link.
As for reserving a seat, It seems like for a small difference we can do first class. In picking a seat for the 4 of us, which is the most comfortable- booth or side by side. It's a long train ride so I want my parents to be comfortable. If we do the booth, do you feel the motion going backwards?