Hi - Heading with the kids for the first time in December 2015! What is the best time to visit each resort? IE is it best to do MK during the day and AK in the afternoon? Or are the crowds OOC at any point?
Okay, first, hello! Is this your first time going to WDW? It seems it may be from your questions. I'll try to help.
I assume you mean "What is the best time to visit each park?" not each resort. The hotels are the resorts.
It needs to be noted that none of the parks can be "done" in a day. Most people (there is some disagreement) would agree that Animal Kingdom and Hollywood studios each take an entire day to tour, while Epcot and Magic Kingdom take 2 full days to tour. So, if you're already aware of this and are planning to visit the parks over several days, and just want to mix things up, then more power to you!
If that is the case then you need to check out some "crowd calendars". These will give you general ideas of the predicted crowd levels on the days you are planning to visit the World, for all 6 parks (this includes the 2 water parks). It will usually also give you info about park hours and the like. Certain people on these Boards have their favorite calendars, but you can usually do a basic search online to find some. You say you're going in December, but did not indicate early December (traditionally low crowds), mid-December (medium crowds), or late December (crazy busy!!!).
Your example, doing the Magic Kingdom in the morning and Animal Kingdom afternoon, is something you could do (provide you had park hoppers), and provided that Animal Kingdom was open later (which it isn't always). So it's not really an advised mix. Doing the reverse would make a bit more sense, however, not if its a Mickey's Very Merry Christmas Party evening (which special event goes on several times per week in December) and you don't have tickets. Point is, there's quite a bit that needs to be considered.
Hope this helps!