Hi Ashlyn! To begin, CONGRATULATIONS!!!! I cannot think of a more magical location to do the gender reveal for your sister! I'm so excited for you...and her...and the future little princess or pirate!!!
We have been to
Magic Kingdom numerous times and there is not one vacation I can think of where we didn't see at least one Cast Member with the clear balloons you mentioned. The only potential hiccup might be if the weather is bad on the day you are there. If it rains hard enough that safety becomes a concern, the Cast Members selling balloons will not be out. I will be sending pixie-dusted happy thoughts for great weather during your vacation.
Because I almost always have a Plan B, I want to offer a couple of other potential suggestions as well (just in case).
You can get Mickey Ear hats in pink or blue embroidered with Mom, Aunt, Baby, etc. Mickey Ears are fun while you are on vacation and make wonderful souvenirs!
Disney Floral & Gifts is another great option. They create the most amazing in-room celebrations for Guests. You can select from
balloon bouquets, gift baskets, or you can create your own basket with their
custom gift builder.
I could not be any more excited for you!! Thanks for sharing your question with the Disney Parks Moms Panel and please let me know if you have any other questions as you plan your magical Walt Disney World Resort vacation!
Kari