Check the
Disney web page of attractions, filtered down for "preschoolers" and "kids". You'll see options that you think they'd like, along with height requirements.
If they're pretty active, don't miss the play spaces in the parks for them to have a little freedom, maybe after lunch. This is especially important if you will not be taking a midday break from the parks..
MK: Casey Jr's Splash and the Big Top Tent (with a/c) are both in Storybook Circus. Also, try Tom Sawyer Island.
Epcot: Green Landing outside Creations Shop (across from Test Track), a seasonal festival playground past Imagination toward WS, and another inside the exit for Mission Space (a/c), beyond the MS gift shop
AK: Boneyard in Dino-land
A lot of ride preferences depend on how tall the kids are, how daring (for coasters, heights, or spinning), and whether they're bothered by the dark, loud noises, or spooky themes. We've had 4 year old grandkids who loved all the thrilling stuff and some who were much more put off by some of these things. When a child is pushed too far, too fast (especially in such a stimulating environment), the end result may be that the child becomes fearful of trying anything else that looks remotely similar from the outside. Not fun.
Besides the play spaces, ALL of our young grandchildren have loved:
MK - Jungle Cruise, Tomorrowland Speedway, Buzz Lightyear Space Ranger, and the Cavalcades
Epcot - Living Seas (everything there) and looking for Remy (rat statues) at each pavilion in World Showcase
AK - Kilimanjaro Safari, Conservation Station train, and Kite Tails
HS - Toy Story Mania, Frozen Sing-Along, and the Cavalcades
The more adventurous kids loved a ton more. HTH.