Hi Leslie!
Congratulations on your trip to Walt Disney World! December is a wonderful, festive time to vacation there. The decorations are gorgeous and there is so much to see and do.
Without knowing how many full days you will have in the Parks, it's a little tougher to make a recommendation. If the four days do not include your travel times, then you could spend one day in each of the main Theme Parks. They all have something to offer for your entire family.
Having two boys of my own, ages 9 and 4 currently, I will list our favorite Parks in order for you:
1. Magic Kingdom
2. Hollywood Studios
3. Animal Kingdom
4. EPCOT
So, if you cannot choose to visit all four, you may choose to go by our list of favorites to decide which to visit.
Mickey's Very Merry Christmas Party is a "must do" event during the Christmas season at Walt Disney World's Magic Kingdom. I highly recommend it, however, you will have to purchase an extra ticket to attend this very popular event.
The weather is an difficult one to predict for this time of year. I've been in December when we could swim in the heated pools at the Resorts, and I've been in December when I needed a parka, gloves, hat, and heated socks - no lie! I would say come prepared with layers that can be added or removed as needed, and toss in the swimsuit, just in case!
I hope this helps you in your planning. If you have more questions, I'd be happy to answer them for you! Have a magical time at Walt Disney World!