You must file a tempory restraining order. Restraining her from moving the children out of state. You may also want to file for temporary domicile custody. This just happened to my son, his girlfriend was moving to another state with his two kids. We immediately went to a lawyer and filed these restraining orders. Because the children were residents of the state you are in, you have a much better chance of doing it NOW!!!! If yo wait more than six months then she will be able to file from the state they are now in. Even though she has already left, it is not too late to do this. She will have to bring the kids back to the state you are now in.
DO IT NOW!!!
DO IT NOW!!!