How it works
I'm going to have to disagree with the prior response. This is how it works:
Medicare is coverage provided to someone that is disabled or 65 (or both). It is not coverage that either parent has on the child, it is something determined by the government and stays with them for the rest of their life (unless they are no longer disabled). So this child apparently has a disability. Part A is automatic, and covers inpatient, hospice, and SNF care. Part B is something they can choose to have or not have, and they pay a premium for the coverage. Most medical care is through Part B.
If there is court-ordered insurance, it is going to be through an employer plan. Unless the employer has less than 50 employees, the employer insurance will always be primary. If it is a very small employer, say 10 employees, then the Medicare can be considered primary. Here is a link to a Medicare publication that explains it:
https://www.medicare.gov/Publications/Pubs/pdf/02179.pdf
If this child has Part A but not Part B, they essentially don't have medical benefits for physicians, diagnostic studies, surgeries, etc. In that case, the employer insurance would be the only coverage, and they would be responsible for the balance after the employer plan.
When someone has Medicare secondary, they almost never have a balance to pay. Medicare decides what it would have paid if it were primary. If the employer insurance already paid that much or more (which they usually do), Medicare says they are not responsible for anything further, and you can't bill the patient either.