Hmmmm. No little boys? I'm saying that because, in my opinion, UO is an amazing place for little boys. They have dinosaurs and superheroes and dragons and magic and Shrek and aliens....all great stuff for little boys.
There really isn't anything geared to little girls, so hopefully your girls aren't expecting the princess treatment. However, my 4 year old niece absolutely adores Universal, so it must be inherently good for both.
At IOA, they should like the Dr. Seuss area, the Sinbad show, some of the Wizarding World of Harry Potter, some of the Jurassic Park island, Storm Force Acceletron and perhaps Spiderman. There are a few fun play areas as well.
At the Studios, they should like Despicable Me, Shrek, parts of the new Diagon Alley area that will have just opened, Animal Actors, the Kid Zone area, ET. They'd probably be fine in Twister and Disaster as well and the older one may like Men in Black.