Considering your children's ages:
Epcot
Must dos:
Journey into Your Imagination Figment ride
Turtle Talk with Crush (in the Living Seas)
Soarin (in the Land Pavilion) (Fast Pass)
Walk around World Showcase and see the different countries
Illuminations (the evening program)
Spaceship Earth (The big ball)
Toon Tag (in Innoventions)
Characters:
Check the map and times guide for times, but the characters arrive on a double-decker red bus in the entrance to World Showcase. The characters come out and the lines begin. You can also find Jasmine and Aladdin in Morocco and Belle and Beast in France.
Animal Kingdom
Must dos:
Its Tough to Be a Bug (a show)
Kilimanjaro Safari (everyone can do this ride!)
Festival of the Lion King (a show)
Mickeys Jammin Jungle Parade
Triceratops Spin (kiddie ride)
Characters:
There is a place just before you walk into Dinoland (on the right) which is where you can find Pooh characters. Mickey and Minnie can be found in Camp Minnie Mickey (where Festival of the Lion King is shown)
Magic Kingdom
Must dos:
Mickeys Philharmagic (show)
Winnie the Pooh
Buzz Lightyear Space Ranger Spin
Its a Small World
Dumbo (this line can get long and there is no fastpass -- if it's busy, try Alladin's Magic Carpet instead)
Wishes (the evening program)
Characters:
You can find Mickey and Minnie in Mickeys Toontown Fair.
MGM Studios
Must dos:
Voyage of the Little Mermaid
Playhouse Disney (if they watch those shows)
Honey I Shrunk the Kids Playset
Muppets 3D
Fantasmic (the evening show)
Characters:
They usually come out in front of the big hat in the afternoon.
Other Tips in General
ASK, always ASK a cast member if you have any questions. Ask them for their opinions, suggestions, etc. Interacting with the cast members can be the best way to experience the Disney magic! Also, remember that you can leave the park and come back. When our kids were younger, we enjoyed ourselves most when we would take a 2-4 hour break in the middle of the day, back at the hotel napping, resting, and eating. Speaking of eating, you CAN take food into the parks. We always take snacks with us so we dont have to buy them there.