If your ex is breaking the law, can you not deny him visitation? I know in the States you can deny visitation to the ex if you can prove that that person is breaking the law and thus putting your children in danger. While they may not be in physical danger (although, with your father they might) the court has already recognized that they are in mental danger and thus issued the court order.
While it would be hard on your children, you have to do what's best for them. How many kids like to be told what to do even if it's for their own good? If the older ones are already showing JW tendencies, you need to act fast for their sake.
Honestly, I'm impressed that you got the court order at all. Do the authorities not pick up the tab in prosecuting someone in violation of a court order?
I would definitely contact your ex's elder body and inform them that he is breaking the law and that you will contact the authorities and every media outlet you can find. Call up the Guardian - they love dirt on JW's - and tell them that your ex is breaking the law with the full knowledge of your parents and the organization. Tell them this is routinely done and clue them in on the 'spiritual warfare' clause which permits lying in court.