If you plan on both kids going on the rides that the younger one wants....am i making sense. For example, we didn't go on Goofy's barnstromer before SpaceMountain. Mistake. DD6 would have loved the barnstormer if we had done it before space mountain, but thought it was boring afterwards....while your 4 yr old will probably love it, your 8 yr old may not (may not even want to go on at all.....depends on the child). We tried to do most of the more Kiddie type rides first, and early in the day, but missed the barnstormer and did it on a 2nd day.
Figure out what your childrens favorite characters are--for DD6 it was Ariel, so we saw her in her grotto (hence not finishing the kiddie rides the first am) and at MGM saw her show--one of DD6's fav attractions.
DD6's favs: (she was almost 7 and 48 in when we went)
Test Track, Fireworks, Space Mountain, Ariel attraction at MGM, Lights motor Action! at MGM, making a mask at Epcot
least favs: the Great Movie Ride--she HATED IT and remembers it's name just so she can say exactly what she hated! (She thought the gangster guy was really "hijacking" the ride, and she was seated right behind him, shaking with fear--calmed down a bit when he saw her and whispered in a non-gangser voice that he was just pretending)
DH Favs
Epcot, food and beer at epcot, Test Track
Me, Soarin (along with DD10, who uses a wheelchair), food at epcot, Crystal Palace, Mickey's Philharmagic--I just loved being THERE in the World!