My dad was in a car accident and he is on Medicare and Medicaid. Is he supposed to report the accident to Medicare and Medicaid?

No, Medicare and Medicaid don't need to be notified. The hospital would have asked for that info when he was in ER. Giving them the number of his insurance carrier means nothing. A hospital will not bill a car insurance company that I am aware of. I would think Medicare would be billed and then Medicaid. If there are any balances due after that, then the insurance company is given those bills by Dad, Dad is reimbursed and he pays the hospital bills. Medicaid usually pays any balances unless a deductable or co-pay is involved.

Dad will receive a statement from Medicare telling him what has been covered. Not sure if Medicaid does. If u get nothing, they probably paid the whole amount.

You can check with Dads auto insurance but I think with anyone, health insurances cover first in accidents and then the insurance company. Or, it has to go thru health insurance companies first, they have to give a reason for non-payment then the car insurance company pays.

I negotiated this myself with my brother and it was a nightmare. You have already a claim for the hospital. So it will BE reported to your supplemental and to medicare. Therefore they will already know. The problem with them paying some of the bills is that the entities and doctors and there will be 1,000 of them like ants to honey, even those who read the xrays, will thing that the car insurance is THE insurance and not understand that there is medicare and medicaid and supplementals, often enough. I had to make more calls than you can shake a stick at to explain why State Farm was not his primary and only insurance, but only a company that would pay "so much" and that other insurance and government had to be billed. Was not a problem with medicare (he wasn't on medicaid) but was with supplemental, and most of all was with the doctors. I still have files thick as an arms breath for all the MDs a year and one half after my brother's death. I wish I could feel safe to chuck them. In fact, second anniversary in May I will do so.
I sure do wish you luck. Expect it to be like being dumped in washing machine agitator for a while if you are the POA. Begin to keep a diary. As I said, if he was hospiitalized they know, but if you wish, it is one call to 1-800-medicare. And don't worry. They will be fine. It is the doctors who will be a nightmare. As I went from POA and Trustee while he was alive to the same after his death mine was (hopefully) more complicated than yours will be. Best wishes out to you.