My suggestion to you is to call the FSS right in DFW with your list of questions. They know their own service better than anyone else and can offer informed opinions about your options. They would know about costs, timing, airport hotels, etc. I really think, under the circumstances, the FSS is your best bet for your parents.

I have stayed in the Grand Hyatt Hotel right in the DFW airport and was very pleased. The escalator to the hotel is right off the airport concourse in Terminal D. The next morning, I went from my room to my Terminal D departure gate in about 15 minutes. The room was really nice and thoroughly sound-proofed.