Hi Shannon!!!
Thank you for your question to the Walt Disney World Moms Panel!! The Fall season/Halloween is such a wonderful time to visit the Walt Disney World Resort!!
The parks are not overly done in Halloween decorations. I would say that it is more of a Fall celebration than strictly Halloween. The Magic Kingdom is the main park that decorates for Fall/Halloween. Even then, the main reason that it is decorated is for
Mickey's Not So Scary Halloween Party. This party is offered select nights during the months of September, October, and November.
The park is decorated just after Labor Day each year in preparation for the Halloween Parties.
Also, the only time you will really see the characters dressed up in costumes out of the ordinary is during the Halloween Parties.
This is truly a great time to visit and I am sure that your three year old won't think that Disney is a Halloween place. I know that our three year old certainly didn't think that.
Thank you again for your question and have a magical day!!
Scott