Are you sure it was purchased in 2017? Expiration dates started in 2016 (had to be used by 12/31/2017) and 2017 tickets had to be used by 12/31/2018.
If it’s an earlier ticket, fully unused, I think you’ll need to upgrade to an adult ticket. You can ask if they’ll allow her to use it, especially if close to the age cut. If a child’s ticket was partially-used but not expired it could be used by the same individual as an adult.
If the ticket is from 2017 and expired, this chart shows the credit value:
https://web.archive.org/web/20170703161001/http://allears.net/pl/ticketchart.htm
Do not expect a “current value” credit towards an upgrade.