Who is the custodian? Why are you not the custodian?

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