See an attorney and ask what can be done in your case. How did your name get on the birth certificates? Were they both born during your relationship? Did you adopt? Did your divorce papers stipulate that you had no children together?
I don't want to give you false hope, but you may have something. If you were legally married when the kids were born and on the Birth ceritificate, you may have more rights that you think. I personally know someone here in Tennesse that we to desert storm. He and wife had one daughter, when he came back 1 year later, she was 6 months pregnant. He tried to understand and forgive her, and signed birth certificate and was going to raise as his and real father wasn't in picture and possibly not known for sure. When child was about 6 months, they divorced and they got joint custody of both girls. Judge said that even though she was arguing the second one wasn't his, that he had accepted it as father and she had also when she allowed him to sign Birth certificate and were legally married at time of birth. She took kids out of state without permission of court, he filed and they ordered kids back to TN and gave him temp custody, went to court and he got full custody. She moved out of state and he has had both girls now for over 12 years. Talk to an attorney.
Disclaimer: I am not a gypsy fortune teller