There are laws for this: Dispute the charge with your credit card company, let them know that you left the room in the condition you found it (if that was the case) and did not authorize any charges to the card.
The hotel will have to prove that you authorized the charge and, if for damage, that it was from your stay, requiring before and after photos with dates to prove that it was you and not a guest prior/after you. The burden of proof is with the hotel, not you. If they don't have proof, the charge will be reversed.
If your kid did draw on the headboard and the hotel has photos to prove it was during your stay, it'll be difficult to argue the amount of $500 - they can charge you for the new headboard, installation, possibly even lost revenue while the room can't be rented due to installation.
Good luck, hope you get it resolved...