Medical premiums in arrears

My ex has refused to pay for half the children's portion of medical premiums which is ordered in our decree. While we were married my employer asked if I would like to be compensated with a higher hourly pay rate or by having my insurance premium completely paid for. My ex and I (while we were still married) agreed to accept the lower wage rate in exchange for the premium to be paid in full. Is this considered an out of pocket expense due to a reduced amount of take home pay??? and should he be responsible for contributing? He is now agreeing to pay for future medical premiums even though the policy is still with the same employer.

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Does he pay child support?

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Kind of odd that he would be ordered to pay "half of the premiums".
Usually one parent is fully responsible for maintaining health insurance on the children, and receives an offset on child support in return.