You should contact the collector in writing and by certified mail, return receipt requested and request validation of the debt. You also have the right to tell the collector not to bother you and the collector is "supposed" to honor that request under the FDCPA.
However, most states follow the common law rule that a parent is responsible for the reasonably necessary medical bills of the other spouse and/or dependents. As such, if your child was a minor at the time of treatment, as a parent, if the medical treatment was necessary, you may be liable for the medical expenses under the "doctrine of necessaries." This would be true regardless of any agreement you would have with your ex.
If you were to pay the medical expenses, you would be able to recover the amount from your ex-husband per the terms of your divorce decree.
By answering this question, general information is provided and no attorney-client relationship is established. For specific inquiries, you should consult with experienced counsel in your area.
Sorry to have to tell you this, but just because your divorce decree says one thing, a medical provider is not obligated to comply with the terms of your divorce. Since the services were provided to your child, you are responsible to pay the doctor.
I would suggest that you contact your divorce attorney for follow up assistance on enforcing the terms of the divorce between you & your ex.
Hope this perspective helps!