Texas law no longer states that a child 12 years old (or older) can decide which parent he or she wants to live with. To tell a child that they will get to choose is never a good idea. The current law allows for a child to meet alone with the judge and discuss the many issues regarding custody. The child will be able to express their concerns and desires but these are not binding on the judge. The closer the child is to 18 the more sway they may have over the judge. However, anyone that tells you "the child is 12, they get to decide" is simply wrong.
Make sure that you have an attorney on your side that is ready to help you navigate the tricky issues of child custody and make sure the right decision is made.