You need to hire an attorney immediately. The fact that she left the children, voluntarily, with you and had infrequent visitations will help your situation. The fact that she ran away with the children will usually upset most Courts (if it was for no significant reason). You need to file for Divorce and also get a Temporary Restraining Order with a hearing. Of course, you do need to have some way of locating her to serve her, but there may be other ways to give her notice of the divorce.
You need to file a divorce immediately and get her served, and go in ex parte for temporary custody orders, alleging irreparable harm and flight risk.
When you are still married, both of you have rights to your children. Only when there is a divorce filed, can you gain orders.
You must act quick; status quo is very much favored in custody cases. If you wait, and the children are with her for a long time, this would be the status quo and you would lose rights.
I would hire a private investigator immediately to locate her and the children and get her served immediately.
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