I personally believe that Universal is Disney on the adult scale, the theming is absolutly there, it is NOT just another amusement park. I have always believed that the Port of Entry at IOA is very similar to Main Street all the small details, but obviously themed very differently...
Now into which is better.
MGM has the Great Movie Ride, Playhouse Disney, Muppetvision, the Beauty and the Beast show, Little Mermaid, all of which would be good attractions for the age group you are mentioning, but MGM for me just comes up way short on the connectivity and "flow" of the park and just the # of decent attractions. You can only see a Man and his Dream so many times (I have seen it 3 times and still love it though!). You could easily get through MGM in half a day.
Islands of Adventure is a thrill park, many of the rides are incredibly thrilling and well... some are scary for that age group, Hulk, Dueling Dragons, Dr. Doom, all fall into this category. BUT, IOA also has such amazing family rides
as Dudley Do-Right which beat out Splash Mountain as the best water ride in our recent magazine survey (Theme Parks Magazine), Seuss Landing, the Flying Unicorn, Camp Jurassic, and the flyer coaster there, Bluto's Bildge Rafts (my favorite rapids ride overall), etc. Its a very very very good park period.
Universal Studios on the other hand is very adult oriented as well, but is a very good competitor to MGM especially now with Mummy the Ride to compete with Rockin' Roller Coaster. Its more movie oriented where MGM is just more Disney themed ride oriented. Meaning EVERY ride at Universal is themed to some movie whether it be Jaws, Earthquake, Twister, Men in Black, Back to the Future, Terminator, etc. Universal Studios attractions migth be on the scary side for children that young though.
Overall its a personal decision I like IOA out of the 3 but many might disagree and that's ok.
Jennifer