Welcome to planDisney, Josie!
While tickets to Disney World are
non-refundable and non-transferrable, you may be able to change the date on your son's 2 day ticket if he's planning to visit the parks again at a later date!
One of my fellow panelists,
Kay, has a great answer explaining how to check to see if tickets can be modified. I would recommend checking that out as a possibility.
If you are able to modify that ticket for your son, you can simply purchase a single day ticket for him and your daughter for this upcoming trip!
It's been a little chilly around these parts, Josie! Take this as your sign to find a cute Disney sweater from one of the
gift shops! It's a functional and adorable souvenir.
We hope to hear from you again! Sending magical wishes for your vacation!
Allyn