My six year old daughter's favorite rides/shows are:
MK: BTMRR (hands down the #1 ride in her book), Splash, all of Fantasyland, Aladdins magic carpet, jungle cruise, HM, Buzz, PotC
Epcot: TT, JIYI, Ellen's Universe of Energy, El Rio de Tiempo
MGM: Muppetvision 3D, Backlot Tour, Star Tours, Voyage of the Little Mermaid (show)
AK: KS, Triceratop Spin (aka Dumbo), FotLK (show)
Have fun!