Hi, Lauren!
My family has been in the same situation as your sweet little Princess!
Since guests 3 and older require park admission and receive
MagicBands, here's what my family did for our then 2-year-old who didn't receive one.
We visited the
Disney Store online, or the
Shop Disney Mobile App, and bought our daughter a special MagicBand. Here you can choose from character MagicBands, which we decided on Tiana. We had her MagicBand mailed to our home and presented them to our children in a special Walt Disney World vacation kit! It included the MagicBand, a vacation t-shirt, and autograph books! It was a hit!
If you want, you could add your youngest's MagicBand to the box that the other MagicBands come in. Just open up the box and add her MagicBand to the rest of the family's before anyone sees!
Once your arrival to Walt Disney World, your family will be all set! We even allowed our Princess to tap her MagicBand everytime everyone else did! This little bit of pixie dust and magic made their trip even more magical.
Wishing magic and memories,
Korliss