Hi Opal,
How wonderful to be visiting Walt Disney World for the first time with a 4 and 6 year old! They are at such a great age for the magic.
Chances are, as you get further away from the winter holidays, the crowds at Walt Disney World will start to diminish a little. Since school children are typically out of school through January 1, crowds should be busiest closest to January 1. I would suggest that you visit on January 4 - that should be the least crowded of the days that you mention. Walt Disney World currently has park hours listed for all three of your possible days as 8:00 a.m. to 1:00 a.m., so you should have plenty of time to do whatever you want on any of the days! If it turns out that you cannot visit on January 4, my second choice for your dates would be January 3, and third choice would be January 2, based largely on crowds. When I am looking for crowd predictions, I visit the unofficial site touringplans.com, where you can see up to a year of Disney theme park crowd predictions for a low cost. I do also suggest that you check the
Magic Kingdom Hours before you go. They might change, so just double check those closer to your date. The link I provided is for January 4 - just use the arrows at the top to navigate to other dates.
I hope that these thoughts are helpful for you, Opal. Have a wonderful time planning your trip, and please do not hesitate to ask if you have any further questions!