First, you need to file an injucntion ordering her not to move the children from the state without the approval of the courts. Even if it is against the law in your state, or it says that she cannot move in the custody degree, the fact is that they will move anyway. Than, by the time you get into court to make a case for why she should return the children back to the state, they have settled in and the judge will not wish to disrupt that. With an injunction, if she does it anyway, you can go to court immediately to file for a change of custody.
Now, you shoudl knwo that the National Organization for Women is taking on these type of cases, to push them to the Supreme Court level. They argue that it is a violation of her right of freedom of movement. And as long as she has primary custody of the children, she has the legal equivilent of ownership of them. Both California and New York Supreme courts have ruled that mother cannot be prevent from moving, regardless of the relationship of the children with the father. They are now attempting that in Missouri. And I can tell you, they have a lot more money than you or any father’s group to fight it.
You have to do your best to make an argument for why they children need you, and that the step-father cannot replace you. You may want to file for full custody of them.
Now, this is what we do at the National Congress for Fathers & Children. We educate you on your rights and how to fight for them. Most especially, you have to learn to collect the evidence, and not rely on an attorney to do it for you. The main thing is you need to move on this right away. You can file the injunction, Pro Se. That will be your first step. I can gaurantee you will likely have to go for custody, so you need to be prepared for that possibility.