First, why are you not the custodian of the account?
Second, buying a new car is likely misuse of custodian fund. Providing transportation for minor is the obligation of the parents, the minor should not pay for his/her own transportation.
Third, investment brokerage provide custodian account service too. The trick is how you can convince the court to transfer to a brokerage account.
https://www.fidelity.com/open-account/custodial-account