We just got back from 9 days. I asked my 4 year old what her favorite parts were. "Peter Pan, play areas, and the Candy Lady" was her response. She likes all the play areas, even Pooh in Fantasyland, which to me seems a little young for her. She also likes Tom Sawyers Island, especially the barrel bridge. Also the Boneyard and AK and the play area at MGM. The Candy Lady is in Japan at Epcot. The times are listed and be there a little early so that she gets one. She will ask what animal and what color, and will then make an animal sucker out of candy and give it to your niece.
I also liked the Family Fun Day Parade and MK. The first day I was there I didn't see it listed, but had looked only under the Parades section. It is listed under shows or activites on the back side of the schedule. Just be in the circle at the front of the castle at the time the parade is listed to start. Then line up behind a character and you are in the parade. The parade actually starts 10 minutes after the time that is listed. She enjoyed being in the parade, but says she likes the play areas better.
Hope this helps.