Man. That's a lot of driving.
Not sure if you heard, but Texas is big.
DFW->Austin 4 hours
Austin->SA 1 hour
SA->Corpus 2.5 hours
Corpus->Galveston 4+hours
Galveston->Jefferson 1.5-2 hours
Jefferson->Dallas 5 hours
Unless they just want to be completely wiped out... I'd say, cut out the trips to Corpus and Jefferson and spend a couple days around SA/Austin (state capital, Alamo, Riverwalk, Hill Country) and a couple around Houston/Galveston (Moody, Seawall, NASA, LBJ library). That way they can hit Brennan for Blue Bell and Shiner for beer. Also get a good taste of hill country BBQ.