I know this is all dependent on the personality of each child. Here is a list of what my DS, age 4 in Sept., likes at each park:
Magic Kingdom:
Tomorrowland: Buzz Lightyear SRS, Carousel of Progress, TTA, Tomorrowland Speedway
Toontown: everything
Fantasyland: everything EXCEPT Snow White's SA and Peter Pan's Flight (both too intense for our son)
Adventureland: everything
Libery Square: LSRiverboat
Note: PoC may be too intense (only you will know), Swiss Family Treehouse requires a lot of stair climbing and walking (use your judgment), don't miss Magic Carpets of Aladdin (it's easy to miss. It's similar to Dumbo but the line is usually shorter), Hall of Presidents is a great, cool place for him to take a nap (although he has been known to stay awake and occasionally enjoy it

)
EPCOT:
Turtle Talk with Crush (don't miss this!), Soarin', Spaceship Earth, Living with the Land (boat ride), El Rio de Tiempo (Mexico),
Note: Our son is fearful of the ride in Imagination. We haven't taken him to HISTA b/c it is probably too intense. Maelstrom at Norway is too intense for him, too. DS doesn't do the circlevision movies well, either.
MGM:
HISTK playground, Playhouse Disney, WWTBAM (of course!!), Star Tours (but he's a Star Wars freak!!), Muppet Vision 3D
Note: DS doesn't like loud noises so we wouldn't have done Backlot Tour with him. Great Movie Ride can be frightening as can IJSS and LMA. Sounds Dangerous is in complete darkness. VotLM has some frightening parts with Ursula (but if your child has seen the movie, it's no different than that.)
AK:
favorites are Kilimanjaro Safari, Rafiki's Planet Watch, Triceratop Spin, Boneyard, Tarzan Rocks, Pangani Exploration Trail.
Hope this helps!