We did a WTX road trip for spring break, ranging from Carlsbad, Ft. Davis & BIBE with our kids (someday I'll finish a complete report). If you've never been out in the region, buy a copy of
Adventures in the Big Bend: A Travel Guide for good guide book of things to see and do.
For the drive out take lots of things for the kids: DVDs, coloring books/notepads, music, travel games. We carried lots of picnic and snack foods and ate on the roadside almost everyday. We drove from Houston to Pecos (headed to the Monahans SP Sand Dunes) and stopped before Kerrville at a roadside park for lunch.
Bring your swimsuits (and snorkel gear if you have it) an be sure to spent a half a day at Balmorhea SP swimming in the spring pool. Its a neat oasis out there in the desert. While in the Balmorhea area, eat at La Cueva De Oso for a pretty good meal (The Oso Flojo Lodge, next door and owned by the same family has nice accomodations if you need a place to stay).

Drive the scenic loop around the Davis mountains using the book to describe the features you'll pass.

Be sure to visit the state park and drive up Skyline drive to see the view to the SE down on Ft. Davis and to the park and McDonald observatory to the NW.

Visit the observatory star party one night for great views of the heavens. Bring warm clothes as it gets chilly at night.

If you want to venture further south, you can spent time in Marfa and Alpine or even make the drive down into the Big Bend National Park.
On the way home, visit the Caves of Sonora and then stop for the night in San Antonio, with dinner on the riverwalk and a trip to the Alamo the next before heading back to Houston.
David