When we go, we spend literally ALL DAY at each park....from open to close. Of course, we don't have small kids so it's easier for us to do that! We stay for a week and get the 5 day passes and do MGM twice only because they have more rides, more shows, and more shops. The best part about this park is that there are a lot of places to cool off - most of the stores connect to each other so you can walk right through.
Animal Kingdom is my favorite park because I'm an animal lover, but I know that kids like it too because it's like a big zoo and there are a ton of characters there. The Safari Ride is awesome so definitely do that with your little ones. And Mickey's Jammin Jungle Parade is great because there are a lot of characters that actually walk along the route instead of riding on floats. Line up early for a shady spot.
Magic Kingdom is huge - Don't rush to get through the whole thing because you will only frustrate yourself and your family. Take things as they come and do all of the areas in order so you avoid going back and forth across the park which wastes time.
EPCOT just started doing Kid Stops in most of the countries in the World Showcase to make it more interesting for the little guys. I happen to really enjoy that park, myself!
All in all, I would say the worst mistake my family ever made was "planning" our days down to the minute. We were frustrated if we had to wait in line because it was taking away from our time to do other things. Don't let the time bother you...Like I said, last year I went with just my boyfriend, we had no plan other than what park to do each day and it was so nice not worrying about "not getting to do everything." If we were hungry/tired/thirsty etc, er stopped. If we wanted to ride Tower of Terror 10 times, we did. Just being able to enjoy the atmosphere and not feel pressured to do a certain # of things really makes the vacation enjoyable.
