Welcome to the Disney Parks Moms Panel, Catherine!
Autumn is such a delightful time to visit Walt Disney World Resort. It sounds like you have an amazing trip planned for your family. I'm so excited for you!
While we can never be sure when the elves will sneak into Magic Kingdom Park to decorate for Christmas, we can look at last year's special events schedule for some clues as to when the magical transformation may take place. For instance, we know that the Christmas decorations will not appear before the last
Mickey's Not-So-Scary Halloween Party is held. That was on November 1, 2017. But the park will need to be dressed for the winter holidays before the first
Mickey's Very Merry Christmas Party. That was November 9, 2017. If Santa's little helpers follow roughly the same schedule in 2018, it seems likely that Cinderella Castle will start glistening with winter lights sometime between November 2nd and 10th in 2018. (Please note that while elves are friendly, talented and creative, they have a penchant for following whatever schedule they please so these dates are just estimates.)
It seems to me that it is unlikely that Magic Kingdom Park would be decorated for Christmas only two days after Halloween in 2018, but it is possible. I will keep my fingers crossed that you will have the opportunity to experience the wonder of both Halloween and Christmas when you visit this fall.
Wishing you many magical memories,
Jill