Hi Samantha and welcome to the DIS boards. The first thing I would suggest is getting a really good guide book....a lot has changed in the past 10 years!!! The 8 y/o boy may have very different tastes as far as attractions go, than the rest of the kids. But, I'll give you my personal favorites (and my dd's faves, she started going at the age of 6).
MK....Dumbo, Winnie-the-Pooh, Philharmagic, Peter's Pan Flight, IASW, ToonTown, BuzzLightyear, Jungle Cruise, Liberty Belle, Aladdins Flying Carpet..now if the kids are daring they can try Splash, Big Thunder and Haunted Mansion.
Epcot...Universe of Energy, the different pavillion areas with the hand's on experiences, SpaceShip Earth. Soarin', Test Track and M:S may be too intense for the younger ones. World Showcase is fun but can be boring for little tykes. Try doing the Kidcot stops. IllumiNations is wonderful but loud!!! Honey I Shrunk the Kids 3-D show is fun.
MGM...Little Mermaid, StarTours, Great Movie Ride, Backlot Tour, Indiana Jones. There is a Beauty and the Beast live show that is really nice. The Muppets 3-D is good. And Fantasmic! after dark but can be intense for the littlest ones.
AK....Tough to be a Bug is cute but can be overwhelming for youngsters, safari is good for the kids, Rafiki's Planet watch, Lion King Show is NOT to be missed, and the walking trails can be interesting.
I'm sure I've forgotten some, but those are our highlights, at least the ones for the younger set. There are height restrictions in place for a reason...it's not based on age but in the way the restraints work so don't try to 'fudge' the kids' heights.
Any others I may have forgotten guys??
I second the idea of character meals if you can fit them in. I hate waiting in long lines for those characters in the parks. But, getting them done while you eat is a god-send.