We first took my son when he was 2 1/2 and he LOVED it.
He's always been into animals, so the Kilimanjaro Safari and the animal viewing trails in Animal Kingdom were right up his alley. So was the Boneyard - the dinosaur playground at Animal Kingdom. He ran around, climbed, slipped down the slides, jumped, etc... and had a blast.
In EPCOT, he particularly enjoyed the Seas pavilion (I think he fell in love with the manatees) and actually really liked the Land pavilion was well. He liked watching the Chinese acrobats doing their thing at the World Showcase, and entertained himself in the red phone booths in the UK pavilion. He also loved the big water fountain, watching the water "dance" to music. (At that age, it doesn't take much!) After dark, the sparkling pathways absolutely fascinated him. He could have spent all night walking on "the stars in the ground."
In Hollywood Studios, he liked the Disney Junior Live show.
At Magic Kingdom, he loved the Dumbo ride (and its queue), the Winnie the Pooh queue (more than the ride!), and the afternoon parade. He liked watching Cinderella's Carousel, though he refused to ride it. The Casey (circus train) themed splash area in Fantasyland was a HUGE favorite - he could have stayed there all day, and we went in late November, so it wasn't even that warm!
In all the parks, he LOVED meeting the characters. He practically melted onto Mickey Mouse and couldn't be pried off. Character meals were a major highlight, as were meet and greets. He watched a few Disney Jr shows, namely Mickey Mouse Clubhouse, and was familiar with many characters, but we talked to him before our trip and explained that Mickey looks small on tv, but was about daddy's size when you meet him where he lives, and that Goofy, for example, is even bigger. He was a bit nervous around Goofy (who's about 7feet tall, I think?), but otherwise took the large characters in stride and loved them. Might be worth mentioning to your little ones, so they don't expect their beloved characters to be no more than the height of your tv!
