Hi, Grelda!
Welcome to planDisney, and thanks for sharing your question with us!
I'm happy you'll get an extra day at the
Disneyland Resort with your son. This is such a special treat at this time of the year.
Your strategy is the best option since you're going in December, which tends to be a busier season. You can
buy a one-day ticket and use it for your son's last day after he has used his original two-day ticket. This strategy also gives you the advantage of
booking a park reservation for that day in advance versus trying to extend your ticket in person.
The Disneyland Resort won't issue refunds in that case. Currently, the
rules state that Disneyland Resort tickets are not refundable, but they also say that you can apply the amount paid for any wholly unused, expired Disneyland Resort Theme Park ticket toward the purchase of a new ticket at the current price so long as the new ticket purchase price is equal to or greater than the amount paid for the original ticket. So if you find availability and you can extend your son's ticket, you can save the remaining ticket for a future use. How does that sound?
I hope this answer helps you make the most of your time with your son, Grelda! Please let us know if you have more questions and we'll be glad to help!
May the magic be with you,
Julie