Hello, Cheri!
Welcome to planDisney and thanks for bringing us this extra joyful question! I'm sending your whole family a huge "Congratulations!" because welcoming a new little Mouseketeer is about as good as it gets. Celebrating is what Disney friends do best! Here are a few ideas for gender reveals in Disneyland Park.
Purchase the baby's first Mickey Ears at The Mad Hatter (there are locations on Main Street, U.S.A. and in Fantasyland) and have the hat personalized with pink or blue thread to reveal the gender. If the gender is still a surprise for everyone in your party you can share a note with the Cast Member doing the personalization and ask them to slip the ears in a bag without anyone seeing them! I would recommend opening them somewhere with a Disney PhotoPass Photographer to take plenty of photos for you.
Shop for two plushes (make sure they're the same price, no peeking!) and then ask the Cast Member helping you complete your purchase to put the correct plush in a bag for you to open later. I've even seen couples take that special bag to their favorite character so that they can be the one to give them the good news. I think this idea would work especially well if you visited Mickey at his home, it's a more private meet-and-greet experience!
Some other fun items for gender reveals include the classic Mickey balloons, personalized ornaments from Plaza Point, and complimentary celebration buttons from Guest Services! Whatever you choose, I highly recommend that at least one person in your group purchases Disney Genie+ on the day of your visit, because that purchase includes Disney PhotoPass downloads for the day. You will definitely want all the photos! And even if you don't involve a character in your big moment you can still share your news with them. They love to celebrate, too!
Hopefully you'll be bringing your newest family member for a Disneyland visit of their own, soon! One of our family's favorite traditions is taking photos of our kids in the same spot each visit. The Sword in the Stone in Fantasyland is our spot, but there are so many great choices! I love that many of the Fantasyland rides like Dumbo the Flying Elephant and the Mad Tea Party have ride vehicles outside the ride that you can take photos in. You could take a photo in one of these spots now during your gender reveal, then take another photo on baby's first visit!
I hope these ideas spark a magical plan for your visit, Cheri! I'm sending lots of pixie dust for a safe, healthy pregnancy for your daughter and many future visits for you and your grandchild.
Keep moving forward!
Amanda