I am sorry to hear you are having this problem.
In order to make a record that you have requested that the children's father pay one-half of the uninsured medical bills, it would be wise to send him a copy of the bills you are requesting reimbursement for and a dated cover letter requesting reimbursement. Save a copy of your request.
If your children's father still does not pay, you will need to file a contempt action with the divorce court asking that the terms of the Divorce Judgment be enforced. You will probably want to hire a local attorney to bring the action on your behalf. If you cannot afford an attorney, you can contact the clerk of court's office in the county in which you were divorced to see if they can direct you to online or print resources concerning filing a contempt action.
Disclaimer: This answer does not create an attorney client relationship and is not legal advice.
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