Hi Amanda,
Congratulations on adding another member to your family! What a blessing!
The first thought that came to mind is to pick up one of those "Big Brother" or "Big Sister" t-shirts for your little one to wear. You could also buy your baby-to-be a plush Mickey or Minnie (they have the "Baby Mickey" and "Baby Minnie" in many of the shops). Then stop at the Town Square Theater on Main Street, USA to
visit Mickey Mouse. There will be a Disney PhotoPass photographer available inside (they will take photos for you with your own camera as well). You can explain to them that you want Mickey to point at your toddlers shirt while you hold the plush (or put your hands on your belly). Have fun with it! Mickey will be very excited to help share your news.
If you or your little one has another favorite character, you could pose with them instead. Simply pick up a Times Guide as you enter the parks. Specific character Meet & Greet Times and locations will be listed on there.
Another idea would be to custom order a cake that says "Big Brother" or "Big Sister" on it for one of your character meals. You could hold the cake up in front of your toddler with the characters surrounding your family and people would have to read the cake to see what you are celebrating.
Hope this helps. Whatever you decide to do, I hope you have an incredible time celebrating your newest addition.
Wishing you a lifetime of magical Disney vacations with your family,
Kaylene