My soon-to-be ex-wife and I have been at an impasse over some medical treatment that our sons have been receiving. It's elective treatment and, frankly, I cannot afford my share of the co-pays. A solution is to cover the children under my and my ex's medical insurance. By doing so, our sons will have primary and secondary insurance, and the secondary insurance will cover co-pays! And guess where the increased health insurance costs are deducted from - wait for it - they come off of my child support! (Or at least that's the case in Washington state where I live. Your mileage may vary.)
Explore this option; your children will win because they get the quality medical treatment they need, not only now but in the future too. Your ex will win because she (or he) doesn't have to pay co-pays either. You will win because co-pays are eliminated or reduced, and because your child support payment is reduced. This approach will also likely reduce opportunities for conflict between you and your ex over medical treatments and expenses.
This is a win-win-win-win! How often do you see one of those?!?