I would look on this website for an attorney. You need to first establish your paternity. It sounds like you have had them the majority of the time for over 6 months. However, you are going to have to prove this in a court of law. I assume that their mother is going to deny your allegations. You need to meet with an experienced family law attorney and discuss your case in person. You can prove your case with written evidence and with witnesses.
If I were you, I would not go to the Office of the Texas Attorney General without an attorney! From the facts you have presented, you need legal representation. The TX A G office will only want to set child support on you. They don't care if you claim that the kids live with you if the mother says they live with her. Their office does not have the time or resources to determine who is telling the truth.
You need to file a counter-suit affecting parent child relationship seeking to be adjudicated the children's father and seeking primary possession. You need an experienced attorney to do this. Go to my website www.wtylermoore.com and see if there isn't more info for you there. I've got questions answered and more info about me.
I am not intending this to be legal advice, because I don't know the particulars of your situation. Call me if you would like to discuss this or other isues.