Okay...I'm torn. Let me give you what I personally feel are the toddler/pre-k highlights at each park, maybe that will help with your decision

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Epcot (probably my 3rd choice for you although I LOVE this park):
1. Turtle Talk with Crush
2. Rio de Tiempo (sp): Cute boat ride in the Mexico Pavillion
3. Journey Into Imagination (it's okay...one "gotcha" moment that still freaks out my 5 y/o)
4. Spaceship Earth: My kids love this ride.
5. Kidcot stations throughout WS
MGM (my 2nd choice):
1. Voyage of the Little Mermaid Show: Must See for Ariel fans.
2. Beauty and The Beast Show: GREAT!! Very entertaining.
3. Playhouse Disney Live! A must-see for pre-K and toddlers who watch the Disney Channel.
4. Muppet Vision 3-D: Cute 3-D show.
AK (My personal 1st Choice):
1. Kiliminjaro Safari
2. Pocohontas and her Forest Friends (cute show)
3. Flights of Wonder (another cute show)
4. Festival of the Lion King: Best show on WDW property IMHO (aside from Cirque LOL). Not to be missed!!!
5. Great character viewing areas in Camp Minnie Mickey. They are all wearing their safari clothes. Very cute....excellent way to see loads of characters!
6. Triceratops Spin: Think Dumbo w/ Dinosaurs. My kids love it. Go figure

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7. Walking the trails (White Tigers, Gorillas, Large Fruit Bats). This is always a highlight for us. My kids love to walk the trails.
I think that AK is a more relaxed park. Some folks don't enjoy it as much but it's a favorite of my kids. To break up the day you can schedule lunch at Rainforest Cafe. THe kids always enjoy that!
You can't go wrong. There is sooooo much to do at all three parks.