It just depends on the kids. My kids love DHS. They could never have made it a half day park at that age. They are now 8 and 10 and we spent a full day, a half day, and a couple of hours there last trip. If they like Pixar characters and Disney shows, and princesses, you can easily spend a full day there. If you go on an evening EMH night, be sure to hit up the Disney Junior dance party. It is fun for little kids who like to dance. DS did it again last year (7 at the time), but his sister (9 at the time) didn't stay long. She and DH went to see the Little Mermaid show again. I think she was just embarrassed by her brother's dancing.
Here is what they loved at that age:
The Voyage of the Little Mermaid
Beauty and the Beast stage show
Disney Channel Rocks (was previously a High School musical show)
Disney Junior Live (DS did it at 7, but it is now getting a more girly lineup, so that was probably our last time)
Disney Junior EMH dance party (only available during PM EMH)
Fantasmic
Animation area
Lights Motor Action
HISTA playground
Muppetvision 3D
Backlot Tour
Pixar Parade
Toy Story Midway Mania
And of course, the various character meet and greets.
ETA: They also love the Great Movie Ride, but there are some scary parts.