How can I prevent ex from taking kids out of state

I know I can force the kids back but can I stop her for taking them in the first place?

We have no agreements or court order over custody or child support. We have been divorced for close to a year now. They have been with her but we’ve had them for the past month(s) due to the summer. We would like to keep them. In short of filing for full custody, can I make her keep them in state?