Yes, they shouldn’t care that it’s an adult ticket and she’s 8. And no, you don’t have to use it first. I don’t know how long you’ve had the ticket but if it’s older the value you get won’t be the current gate value. Once they switched to tickets with expiration’s they began locking in the gate price for existing tickets. If it’s a newer ticket then the value will be based on the dates it was purchased for.