Big Bend and Ft. Davis both are great. Lots of great places in the Ozarks, too. The nice thing is that kids that young are usually happy to hike and throw rocks, you probably don't need to do a lot to entertain them.
Draw a 450 mile circle around Belton and you have a ton of opportunities. In addition to the previously mentioned, Caprock Canyons and Palo Duro are in my direction, though toward the end of your range.
Copper Breaks is closer to you, but far from town and great mainly for solitude and star gazing.
Abilene State Park isn't spectacular, but they have star parties there, too. It's also close to Perini Ranch and there's a nice little museum at Buffalo Gap. The Zoo at Abilene is supposed to be good.
Lake Amistad is spectacular, in a desolate way.
Wichita Wildlife Refuge in Oklahoma is interesting, and there are some nearby attractions like Lake Altus, Meers Store, the old Ft. Sill parade grounds and museum, the artillery museum at Ft. Sill, Mt. Scott....
Post again if you want me to bloviate on any of the above...