We have two boys. The oldest is 4 (5 in Aug., 45 " tall), the younger just turned 3 this month and they've been to DL 3x now (Oct. '10, Oct. '11, and Feb. '12).
Our 4 yr. old has never been afraid of the characters (with the exception of The Mad Hatter!). He STRONGLY prefers the outdoor rides. His faves: Dumbo, Teacups, Casey Jr., the carrousels at both parks, Autopia, Astro-Orbiter, and Jungle Cruise. The exception when it comes to indoor rides would be Ariel's Undersea Adventure, The Many Adventures of Winnie the Pooh, and Buzz Lightyear's Astro Blasters. Loves those. He likes to ride the
Disneyland RR and Monorail, too. And really enjoyed Disney Jr. Live on Stage at DCA.
He's ok with Peter Pan's Flight and Alice's Adventure's but buries his head for all the other Fantasyland dark rides. Doesn't care for Monsters, Inc. He says he doesn't like Pirates (the drops, lightning, Davy Jones in the mist) but there are parts he does enjoy. Did NOT like Silly Symphony Swings. Did BTMRR once and was a trooper but refuses to go again. So things like Space Mtn. and Splash are totally out of the question for now. Overall, our 4 yr. old likes his attractions tame, well-lit, and non-threatening.
BUT every kid is different!! He has a younger cousin and she can't get enough of Space Mtn. Go figure...
Have a magical trip when you go!