Hi Olga!
Welcome to planDisney! First off, congratulations on your upcoming Mouseketeer!! How exciting, and chEARs!!
No one loves a reason to celebrate anything and everything more than Mickey, and I can think of no better place to create this core memory moment than at The Happiest Place on Earth! I'm so sorry to say that outside balloons, whether already inflated or not, are not allowed in Disney's theme parks. I also checked about the celebration powder, which is usually colored cornstarch, in case there was some other way to keep that part of your reveal; when reading over the posted
Disneyland Resort Rules, there isn't anything specifically listing that you cannot bring it in. There is a line, however, prohibiting "fireworks or other similarly explosive and/or flammable objects, smoke machines or fog machines" which I know this isn't but I'd hate to risk it.
However, don't fret yet because I have a few ideas. First up is if you're planning on sharing this epic moment with more loved ones present, instead of bringing your own balloon you could have someone in your party purchase a pink or blue iconic (ear-conic?)
Mickey balloon from a vendor and deliver it to you with your eyes closed while a
Disney PhotoPass photographer captures the magic. Another idea is you could select two different newborn outfits or sweet stuffed animals - like a baby Mickey or mini Minnie - and ask the Cast Member at the cashier stand to only charge you for whichever one coordinates to whatever is inside the card from your doctor's office; then when you're ready to find out, you'll open up your shopping bag for your best souvenEAR yet!
If you really had your heart set on the popular powder balloon reveal, I've recently seen table-top bouquet-sized options where the mystery balloon is inside a bigger clear balloon, so once popped the powder stays contained. You might be able to bring one of these centerpieces to a restaurant located in a Disney Hotel or in the
Downtown Disney District as they fall under slightly different rules than the parks. (Your best bet would be to call the downtown restaurants and ask if balloons are allowed as they are all owned by separate third party operators.)
I am so sorry, Olga, that this answer wasn't as Up-worthy as you initially dreamed it to be. But I am very confident that you'll come up with something that you love even more that is filled with just as much Disney Magic! Congratulations again, and We'll See You Real Soon!
~Kristen