Hi Rachel!
I have two suggestions:
The first is the Animal Kingdom Lodge, my 5 year old son's favorite resort. Besides seeing animals right outside the resort, they have an outstanding pool at
Jambo House, the main part of the resort. You are also allowed to pool hop to
Kidani Village, the DVC part of the resort, because Jambo House and Kidani Village together make up the Animal Kingdom Lodge. The pool at Kidani Village is like a mini-water park. Young guests love it.
My second recommendation is for either Disney's Beach Club or Yacht Club Resorts. I personally prefer the Beach Club, but they both share
Stormalong Bay. Storamlong Bay is absolutely fabulous, a mini-water park and in my opinion, the best pool on property. It is a sand-bottom pool with an incredible water slide and a mini-lazy river. There's actually a sand island in the middle of the children's pool area where children can make sand castles. My 5 year could spend an entire vacation in Stormalong Bay.
Both resorts offer other activities for young guests. Be sure to pick up a guide when you check-in!
By the way, I don't think you can go wrong with any Disney Deluxe Resort! They are all kid-friendly, including Disney's flagship, the Grand Floridian. Thus if you are going to be spending a lot of time in the Magic Kingdom, you might consider staying in a Magic Kingdom Resort. That will make it easier to return to the room for an afternoon break, nap, etc. My personal favorite is the Wilderness Lodge. While it is not on the monorail line, there is fast boat service between it and the Magic Kingdom.
Have a magical vacation!