What is the name of your state (only U.S. law)? Texas
Leave as in go visiting family or go on vacation or leave as in permanently relocate?
If it's the former, she's free to do so unless there's a court order which specifically prohibits it. Unless there's more to the story, taking the kids on vacation is not unusual, nor particularly troublesome.
If it's the latter, then you can go to court to ask the judge to order the children returned.