We accidentally brought too many adult tickets and not enough child tickets, is it possible for a 13 year old use a adult ticket?

You can, but it would be more expensive. I would recommend stopping by guest services. While not guaranteed, they may refund the difference in price. Typically refunds are not given, but you're not refunding the entire ticket, just downgrading to the appropriate one. They have gotten VERY guest-centered in recent years, so it's a possibility. It doesn't hurt to ask. Just remain polite, and dont act like they "have to" (because they don't).

I would call before going, i can't see why not

Honestly, a 13 year old would probably use an adult ticket anyway.

I think I really don't know