There are no rides that a child can ride, but you can't. That's the great part - truly a family experience.
Mine have been going since they were 4 mos old (youngest now 11) and they rode every ride at that age, unless there was a height restriction. They rode Haunted Mansion and Pirates of Caribbean too. They were never afraid at anytime because we took them on young, they were with us and it was great fun.
One thing I did when they were young was get the tape/cd of the music from the rides. So by time we went they knew the words to the Pirates, Tiki, Haunted Mansion, Small World, etc. When they got inside, very familiar.
MK will also have shows like Tiki Birds, Country Bear, Goofy Jamboree etc they'll love.
MGM they can do several shows like Bear in Blue House, Ariel, Beauty & Beast, can ride Great Movie Ride. Stunt show may be intense, sit at the back. Not sure what is now on hold during the construction.
AK they can do the shows like Lion King, Tarzan, the safari ride, probably the dinosaur ride like dumbo. Should love the dino playground.
EPCOT they will love the Living Seas, Imagination, they can ride in the Land, and others.
Don't forget to use child swap so you don't wait in line twice. At each ride ask how to do it, it varies. Mostly you wait together then swap off while the other rides. Works great.