Hi Karen and welcome to the Disney Parks Moms Panel.
You will find that the overall crowd levels will be fairly low to moderate for the first week of your stay at the Walt Disney World Resort. According to the Crowd Calendar on the unofficial website www.touringplans.com, the overall crowd level for 8 December 2016 - 16 December 2016 is 5/10 or less. (This is probably on a par with what you have experienced during previous visits in October and early November.)
You will find the parks will get gradually busier from 17 December 2016 onwards and according to the Crowd Calendar on the Touring Plans website, the overall crowd level is 9/10 from 19 December 2016 - 22 December 2016.
Generally speaking, the week before Christmas until just after New Year is one of the busiest times of year to visit the Walt Disney World Resort.
As far as
Mickey's Very Merry Christmas Party is concerned, I would say that 13 or 15 December 2016 would probably be your best choices. Both of these dates are mid-week (so locals will be less likely to attend) and before the crowd levels start to increase on the run up to Christmas. (You will most probably be too tired to make the most of the event during the first few days of your trip due to jet lag.)
Wishing you a magical holiday filled with memories which will last a lifetime!
Emma