I have been at POP several times over the past few years in several different sections and have not found any of them to far from the main building or the Busses. The Art of Animation is a carbon copy of the POP layout of buildings.
AoA will cost a little more than Pop but I'm sure a 4yr girl would LOVE a little mermaid room. I'm not certain but I would say size wise POR & POP are about the same as far as walking distance from the farthest rooms to Busses or the Food service area. Keep in mind that POR is the Smallest of all the Moderate Resorts. The Values have more guest because the buildings are 3 or 4 stories, where as most of the Moderates or just 2-3 stories. So the place where POP or AoA will be busier is the Food Court; after 8am. Also the Bus stops can seem pretty hectic but move pretty well in the morning. Coming Back after Fireworks, Parades or Closing can also seem to take a long time but I think Disney does a great job with their Transportation Services.
PS. From what I had read, they have fenced off the AoA main Pool so it can only be used by guest there. You can still go over and walk around the areas and take All the Pictures you want.