Hi Rachael,
With the Art of Animation being a new resort, I think it will fill up quickly especially if you are actually visiting anytime between Christmas Day and New Year's Eve, which is one of the most popular times to visit WDW. If you are visiting the first few weeks of December, you shouldn't have to feel as rushed to make your reservations since that is when the crowds are lighter.
If it helps with your decision making, I am partial to the Art of Animation since you will have your 3 year old with you. Our first trip to Walt Disney World was with my sons, who were 4 and 1 at the time. We stayed at the All-Star Movies which was perfect for them with the movie theming and icons throughout the resort. When I've looked at
pictures and videos of the Art of Animation, I'm just amazed at the theming and decor; it looks like it will be pure magic for little ones.
I'll point out a few other features of each resort that may help you with your decision. The Art of Animation has a kitchenette, two bathrooms, & 1 bed as well as convertible sleeper furniture. The Cabins at Fort Wilderness have a full kitchen, one bathroom, and 3 beds (one double, bunk beds & a pull down double bed). Regardless of which you choose, I don't think you'll have any regrets.
Please let us know if you have any more questions; we are happy to help.