We will be in the parks at the same time as you! Here are the activities that my son adores. He is now 10, but these have been his favorites since he's been 5 years old:
Lego Store in Downtown Disney
Star Tours (including the gift shop at the exit full of Star Wars items)
Jedi Academy (be sure they stand in front and jump a lot to be chosen)
Tom Sawyer Island (run around with no lines--yea!)
Buzz Lightyear
Thunder Mountain Railroad
Splash Mountain
Pirates of the Caribbean
Silly Symphony Swings
Any restaurant that sells pizza/flatbread (but especially Trader Sam's)
His favorite full-service restaurant is Cafe Orleans (mac and cheese)
Churros!
He hates Space Mountain when the Ghost Galaxy overlay is there--it is pretting creepy to see the "man on fire"--he still won't go on it at 10 during Halloween season.
Of course, these are his pre-Cars Land favorites. It wasn't there when we last there in December.
With four days in the park, they should be able to try most everything!
If your kids will get up in time, consider arriving at the park when it opens before the hoards arrive. This time we'll first go to whichever park has early entry since we're staying onsite. This is a big change--we use to always go to
Disneyland in the morning then switch to DCA in the afternoon. Cars Land has changed traffic patterns so that's no longer an effective way to avoid the masses.
Have fun!