Hi Kurt! You are going to love visiting Walt Disney World Resort in December! We have been several times during December and it is always magical. The decorations are beautiful and there are so many great holiday celebrations!
My best advice is...plan to layer your clothes. Typically, you will find a low in the mid 50's and a high in the mid 70's. During our December vacations, there have been some trips where we were bundled up in coats, gloves and scarves, while other times we were in shorts and t-shirts and actually had some pool time. On most trips, we have worn shorts and t-shirts with a sweatshirt or pull-over in the morning and evening. When our kiddos were your daughter's age, we had layer upon layer upon layer of clothing so they were comfortable throughout the day. In the past, we have watched the weather and made the call on what clothing to pack a few days out. There were a couple of times that despite our best efforts, we ended up buying apparel at Walt Disney World Resort. (Bonus souvenirs!)
A couple of tips: Make sure you have sun screen for both you and your daughter as it is typically very sunny in December. Also, all 4 Theme Parks have
Baby Care Centers and they are a great place to take a quick break or change her when needed.
I couldn't be more excited for you! To be able to visit Walt Disney World Resort at such a magical time and see the theme parks through your daughter's eyes is awe-inspiring. Enjoy every magical moment!!!!
I hope this helps you with your planning. Please let me know if you need any other information as you plan your Walt Disney World Resort vacation!
Kari