Forgot to answer your question about attractions not to miss.
MK- My toddler's favorite rides were POTC, HM (although those 2 may scare some young ones due to the limited lighting and theme), LM ride, phillharmagic, Magic Carpets.
Epcot- Mexico ride (never a wait when we went), FEA, the play area after the Figment ride, the Seas aquarium area. We had amazing character interactions at Garden Grill.
AK- Triceratops Spin (rode repeatedly with little to no wait), he loved the Lion King show and just exploring the trails.
HS- TSMM, LM show, Fantasmic, Disney Jr show, watching the ducks by the fountain.