My DD is 8 and loves all of it, with the exception of ToT--- doesn't like the drops! She will ride RnR until she can't stand up, though! I think the MGM is skewed a bit older, but that is ok... not every park needs a Dumbo ride, and kids need to learn to appreciate other aspects of Disney.
There is plenty for those ages:
for the 4 yo there is Playhouse Disney, Honey I Shrunk the Kids, Little Mermaid, Beauty and the Beast, Backlot Tour, plus the parade.
For the older ones... they should enjoy just about everything, with the possible exception of One Man's Dream and the Animation Tour.
Don't skip a park because someone thinks they won't like it. They won't know until they try... and also, because you want to do it, is reason enough!