Responded to your other comment already! Theft by deception is different than grand theft auto.

A valet driving off with your car would also be theft by deception, the crime you are describing. This isn't a crime in every state, but there is no state where you can be charged with grand theft auto if you have the keys unless you also committed robbery/breaking and entering to get the keys. That "permission" you discussed is what eliminates culpability on the driver's part. You can't withdraw permission anytime you want and make it a crime.

You're absolutely right! In the states where theft by deception is chargeable, there would still be a gray area because of the intention of pawning vs selling. Op admitted in their post that they knew their parents would not ever get the items out of pawn, which means they knew there was a risk of losing it overall either way. I am in no way sticking up for the parents, just stating what case an attorney may be able to make in regards to that specific charge.

Unfortunately from a legal standpoint, this would be a civil case. He willingly gave the coins to his parents, even if it was under false pretenses. So for example, if I ask to borrow your car to go around the block and get permission, but really I drive to the next town over, you can't report the car stolen. I have the keys, and gained possession legally. That removes it from police hands, and moves it into courts jurisdiction.