Hi there Neftali, and welcome to the Disney Parks Moms Panel!
A visit to the Walt Disney World Resort is a practically perfect way to celebrate your niece's birthday! I'm an early January girl myself, so I've been in the Theme Parks during that after-Christmas time, too. My birthday happens to fall during the Walt Disney World Marathon Weekend, which will take place January 6-12, 2016. When we visited for my birthday earlier this year as the Marathon weekend was underway, we did see lots of Christmas decorations still gracing the Theme Parks. Most of the holiday decor at the Walt Disney World Resort hotels will probably come down soon after New Year's, but with your early visit you may be able to catch the very last vestiges of Christmas there, too.
There's been no official word on when the Christmas decorations will come down in the Theme Parks, and do be aware that plans can change from year to year. But using last year's schedule as a guide, I think you have a good chance of enjoying some of the amazing Disney holiday spirit during your visit.
I hope this helps with your planning, Neftali. Please stop by and see us again if you have any other questions. And wish your niece a very Happy Birthday from all of us at the Disney Parks Moms Panel!
Keep Disney Dreaming,
Kathy K.