With kids those ages, I would definitely hit MK on an early entry morning, which would put you there on Sunday. That way, you can ride everything in Fantasyland, with little to no wait. Well worth it.
Friday: MGM
Saturday: Animal Kingdom until 4:30/5pm/MK for Spectro and fireworks. Try to get there around 5 pm. That way, you will still get to see the parade at AK (a must see), but will get to the MK in time to do a few things, and get a place for the fireworks. You will definitely want to position yourself to watch the castle prior to the fireworks, because there is nothing like the look on a kids face the 1st time they see Tinkerbelle fly from the castle. The bridge between Main Street and Tomorrowland works nicely! She will fly right over their heads!
Sunday: MK-- early entry... we did this, and never found the park to be overly crowded all day.
Monday: Epcot, and if they get bored, you can take the monorail back to MK, or the boat to MGM.
Tuesday morning: back to MK, if you have time.
Have a great vacation!