My DD was that age last April when we went. Her favorite by far was Mickey's Philharmagic at MK. She also liked the Lion King show at AK and the Ariel show at MGM. Spectromagic was a big hit and so was Fantasmic. I thought she might be scared, but she has seen Sleeping Beauty and many of the other Disney films, so she was fine. As for the other rides, we took her on Pooh, POTC, HM, Peter Pan, Snow White, Small World, Buzz, Jungle Cruise and People Mover in MK. She was a little nervous about HM. She didn't like the heads popping up at her.
At MGM she liked the Muppet 3D.
At Epcot, she enjoyed Imagination and Spaceship Earth.
And of course she loved the Safari at AK. We got there at park opening and she was very impressed that Mickey welcomed her and that she got to be part of the parade all the way to the safari.
What she really liked, which I didn't expect, was meeting the characters. She is normally pretty shy, but she just lit up when she got her turn with Mickey. She also met Minnie, Pluto, Donald and Goofy. The pictures are really great...she is just so fascinated. Epcot is a great place for character meet and greet because it is much less crowded than MK. We waited for Mickey for about 20 mins in MK, but we met all the other characters in Epcot and it took less than 20 minutes total.