Hi, I'm Heather, an attorney with 15 years experience, and I'd like to assist for informational purposes.
Many decrees of divorce state that the parties are to keep each other notified of where they are living and changes in address. If your divorce decree states this, she would be in contempt, assuming she's living with this person. In addition, some decrees provide that if a party is traveling with the kids, so if she's just traveling and has not moved, if your decree states that, she 'd be in contempt of this provision as well.
If it's her parent-time, just because she did not follow through with the agreement to give you the kids during her parent-time does not means she's in contempt of court, because she's not violating the court's order; rather, she's violating her agreement made outside of the court's order.
However, if you are worried, you could call the police and file a missing person report for her and the children. It would document what is going on, would get the police searching for her and the kids, and would help you get to the bottom of it. If you're worried about your kids and have called and called, and she's a no-show, you don't know that something awful has not happened and that might be the direction you should go as a concerned parent. If nothing has happened, having the police report will make her look very irresponsible. But you might consider calling her family members first in case they might know where she is.
Has that answered your question?