First, I recommend that your friends check out a book called
Walt Disney World With Kids by Kim Wright Wiley.
Second, here is my opinion:
character meals
They should be prepared to spend a long time with the "interactive" fountain at Epcot...and have the kids wear clothes that are OK if they get soaked. Let them go barefoot, as wet socks and shoes are no fun.
Hopefully, they will stay onsite. They definitely want to take a break midday to go "home", swim, relax. Otherwise they will have cranky kids and the trip will not be as much fun. Cranky grownups too!
If they are going to take the kids on "scary" rides, they should work their way up. For example, start with Goofy's Barnstormer before BTMR. Haunted Mansion before Alien Encounter. They should think twice before taking the kids on AE at all!
Donald's boat at Toontown is a must do.
They should be prepared that the kids will enjoy the fireworks shows, but not the wait before. And Illuminations and Fantasmic are a little long for a 6 year old, he may get bored before the end. Or maybe fall asleep!
Hope that helps!