Welcome back to planDisney, Shari! I'm so glad to be able to answer your question today.
What a sweet surprise idea! This is peak Disney parent magic. The short answer is that if you and your daughter are linked in Family & Friends in
My Disney Experience, and especially if she's the one doing the planning, there really isn't a reliable way to "hide" an extra park day in the app. When you're linked, she will be able to see the
ticket assigned to you both, and any park plans you make will surface on her end. So, the stealth route in My Disney Experience is tricky.
Here's how I'd play it while keeping the sparkle: don't draw attention to the ticket details, and cross your fingers that the change goes unnoticed. Make the change to the ticket as close to your visit as possible, or you can even walk up to a ticket window or
Guest Relations and do it there once you've already arrived! You won't be able to make Lightning Lane selections for that day if you wait to extend your ticket, but it will preserve the surprise for as long as possible.
So the bottom line is there isn't a true "hide my plans" option when you're linked, especially if your daughter is doing the bulk of the planning, so your best bet is to keep quiet, keep calm, hold off on your purchase as long as possible, and spring the surprise in person. Cross your fingers, and here's to the most magical "one more day!" ever.
Have a magical day, Shari!
Jessica