How old are your kids?
Has it occurred to you that they may have reached an age where they have better things to do than talking to parents?
Understand that no court order against your ex is going to compel your kids to communicate with you. How do you think you will come across when Mom says "Daddy took me to court and the judge says I have to make you talk to him or I'll be in trouble"?
I suggest another approach. Next time you visit your kids, talk to them about keeping in touch and try and find out what's going on in their lives that prevents them from communicating with you and if there is anything you can do for them to facilitate contacts.
Nothing stopping you from texting them or skyping them or sending them emails. If you don't get replies it's likely not your ex's fault.

