My ex wife was in a similar situation. Her former husband took their kids from Ga to Fla one weekend during his visitation. The agreement was that he could take them for summer vacation to visit his family in Fla dn was to return them to our house in GA one week before school began. He never came back.
Contacting the local police as well as the police where he was did no good. Two states, different rules. We contacted the FBI as it was kidnapping. But they said it was not being she gave him permission to take the kids. No help.
Finally, I decided to gather court papers showing her custody and we drove down to get the kids. He hid the kids with lines like " they're off with family to Miami" or " My mom is on her way to your house with them". This went on for months.
We knew the kids would be in school there somewhere and made the trip back and at 3am on a school night, called the police to report a kidnapping at his mother's house and met them there with the court papers. The kids were handed over right then. He couldn't hide them from us and had to allow the cops in his mom's home to inspect the situation.
Sometimes, you have to play unfairly because the law isn't always on your side.