Hi Beth!
It entirely depends on you and your family's capacity to deal with crowds. Any of those days are going to be busy, due to time of year, as well as school being out for most US children. Of those days, the "Mega Busy" days are Christmas, the day after Christmas, the 30th, 31st, and January 1. Magic Kingdom, of all the parks, will certainly be the busiest. So, if you had 4 days, with pre-school aged children, I would choose: 2 trips to Magic Kingdom, 1 trip to Animal Kingdom, and 1 trip to Hollywood Studios. Less busier days are 21st-24th, then the 27-29th, compared to the other days of your stay. So you can select from those.
But again, if crowds doesn't bother you that much, it's really hard to avoid one of the parks, particularly on Christmas Day! Everyone is in a good mood and looking to have a good time. Last year, we debated going to Magic Kingdom the day of Christmas, and ended up there anyway. Yes, the park had to have been close to capacity, but it still was a fun experience. In the end, it's what works best for you...
Have a terrific time at the House of Mouse!