Thanks for stopping by with your question. We at the Disney Parks Moms Panel love to help Guests with their planning. Theme Park Admission Tickets are valid for 14 days after the first day of use. Unfortunately, it isn't possible to add days to that expiration window. You may want to consider purchasing 2 separate tickets...one for the first part of your trip and one for the end. You can try a few different options online to see which way would be less expensive for you. I can't think of anything better than 10 days at Walt Disney World...I'm so excited for you!
Please let us know if we can help you with any additional planning. Have a magical day!