My ex husband refuses to abide our divorce decree, what can I do ?

My ex husband is $12000 behind in child support, works for himself so nothing is reported. He picks up on his weekends but returns whenever he wants. Thanksgiving was my holiday , I let the girls go with him for the weekend , he was supposed to return by Sunday @ 6, its Tuesday and he still hadn't brought them home. He told our daughter to take her laptop that isnt supposed to leave my house , I only found out after she was gone she had hid it in her back pack and took it. I'm so sick of this and worry what I'm going to do if he doesn't bring them home. I've called the police and made report but they said its a civil matter and won't get involved. I don't know what good my court orders are if they won't b enforced, and I'm the only one abiding by them. We have joint custody , with myself having the sole right to residency. He has standard visitation. What can I do ?