A quick google search came back with an article from 2014 stating: Until they establish residency, they face
out-of-state tuition and fees that their
GI Bill benefits won't
cover, often costing them thousands of dollars. ... Public colleges and universities set lower in-
state tuition rates because most states subsidize education costs for their residents.
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=10&cad=rja&uact=8&ved=2ahUKEwiU5Z3As4HeAhUMuVMKHcKDAN4QFjAJegQICRAZ&url=https%3A%2F%2Fwww.stripes.com%2Fnews%2Fveterans%2Funderstanding-new-in-state-tuition-ceiling-on-gi-bill-1.301424&usg=AOvVaw0DKysq1aFE0j-l3-KecjEYThis rule may have changed since then.
Edit: I think that as long as your kids are residents of Texas, that is all that matters. I gave may GI Bill to my daughter, and she went to college in both Vermont and Texas when I was stationed in those states. We did not have to pay any additional fees/tuition either time. But, I was also labelled a "Gold Star" level (or something like that) and that gave my daughter additional benefits over a regular GI Bill user. During that time both my daughter and I claimed Texas residency. I would think that if your kids were Texas residents when applying, then In-State would be the tuition rate.