I know that nothing anyone can say will make this situation any better or easier for you but I want to say I am really sorry this happened to you and your family. I also am from Texas and have found some state laws about owning dogs that I think you may find helpful. I do believe you have good chances of winning any lawsuit since I came across this.
Texas Consolidated Dog Laws
Under the section titled "Dogs or coyotes that attack animals" (2c) is what you will want to show a lawyer if you do pursue a lawsuit. It shows that the owners are responsible for all costs and damages done by the dog.
Good luck and I hope this helps you. Again...sorry for your loss and if you need anymore help let me know. I know a lot about dog laws for Texas because my neighbors down the street have some problem dogs and I constantly am looking them up.