Not sure where in Texas you'd like to go or how far you're wanting to drive. If you decide on an area you can google free things to do in that city.
In Fort Worth the botanical gardens are beautiful. Twice daily they do a cattle drive in the stock yards. There are several Hawaiian Falls water parks in the area of Dallas. They are smaller water parks and you can get coupons online.
Austin is a nice city with a huge lake where you can canoe or kayak. The parks there are amazing. There's also a spring fed pool that's a constant 68 degrees. Barton Springs Pool is the name of it. There is moss on the bottom though and it's quite slippery. You could tour the capitol building. If you decide to go there it is cheaper to stay in Round Rock, Cedar Springs or one of the more outlying areas.
http://www.texashighways.com/travel/item/6740-the-big-spl...
Found this link that you might enjoy.
Have a great time with your kiddos.
Blessings!
L.