Let me preface this by saying I don't know mental health billing at all, have no experience with it.
That said, I do know that you bill the insurance provided listing the patient as the patient, and the insured as the insured. So if the parent is not the patient, but the insured, their name should not go on the CMS-1500 as the patient, they would be listed as the insured only.
If the provider is seeing the parent first, before they see the child, who WILL be the patient, then it may not be an insurance billable situation, it may be self-pay, but I don't know for sure...it just doesn't seem right to bill the parent's insurance, listing them as the "patient" for that first meeting if they are not the actual "patient". Anyone else have some insight?
Additionally, if Medicare is the insurance, you absolutely cannot bill for visits with family, only for visits with the patient.