Lori:
Thanks for coming to the Mom's Panel!
When we've gone in December near the holiday, we have found that the days at Magic Kingdom that host
Mickey's Very Merry Christmas Party (MVMCP) are actually less crowded than the days when the night party isn't held. My theory is that there are a good number of people who will only visit the Magic Kingdom when they can see the night parade, and if they aren't purchasing a MVMCP ticket, they'll need to go on those nights when it isn't held to see the parade and fireworks. Coupled with this is the fact that those who typically hold a MVMCP ticket may use their regular theme park ticket to visit Animal Kingdom or Hollywood Studios with shorter operating hours on the same day or are locals who only come for the party.
The exception for crowding is the short span of 4 p.m. to 7 p.m. at the Magic Kingdom on the evenings of MVMCP. In the past, Disney has allowed folks who arrived early for the party to enter the park and so the crowd begins to gradually swell until the evening when day-visitors-only depart.
As for visiting the parks earlier in your vacation week, I would recommend that. The crowds in the park get larger the closer you get to Christmas with some of the highest attendance numbers of the year. Save the bigger parks (Epcot and Animal Kingdom) for later in the week. You'll find that its larger acreage can disperse the crowds easier than the smaller parks (Magic Kingdom and Hollywood Studios).
Wishing y'all a magical vacation!
Whitney