I would suggest Spiderman if your youngest is tall enough. The height restrictions seem fairly high for most rides so make sure before hand:
Revenge of the MummySM - Must be at least 48"
Jimmy Neutron's Nicktoon Blast - Must be at least 40" (persons under 40" can experience this attraction from an adjacent stationary location.)
MEN IN BLACK Alien Attack - Must be at least 42"
Woody Woodpecker's Nuthouse Coaster® - Must be at least 36"
The Simpsons Ride Must be at least 40"
Jurassic Park River Adventure® - Must be at least 42"
Pteranodon Flyers® - Must be at least 36" (36" to 56" must be accompanied by an adult)
Dueling Dragons® - Must be at least 54"
Flying Unicorn® - Must be at least 36"
Dudley Do-Right's Ripsaw Falls® - Must be at least 44"
Popeye & Bluto's Bilge-Rat Barges® - Must be at least 42"
Incredible Hulk Coaster® - Must be at least 54"
The Amazing Adventures of Spider-Man® - Must be at least 40"
Doctor Doom's Fearfall® - Must be at least 52"
The High in the Sky Seuss Trolley Train Ride! - Must be 34". (Children 34"to 48"must be accompanied by an adult.)
Hulk and the Mummy are most likely too high for your 6 year old, unless he is really tall. They do have child swap for both of these and for other rides, if the adults are interested. They are both great coasters! Also, don't miss the 8th Voyage of Sinbad show - it was really funny with lots of action. My nephews really enjoyed it, and we stayed after the show and they took photos with the characters.
If they are really into superheroes may sure to get the schedule of times and locations. Nephews also got really cool pics with Spiderman and Cyclops.
Smart move to be there before opening. As someone else suggested, the best way to tour is to stay onsite - front of the line access is totally awesome. But it is quite expensive!
Have a great time!