I think your 4 and 6 year old will find lots to like at both USF and IOA.
At IOA, there's a whole section devoted to little kids, Woody Woodpecker's KidZone. Feivel's Playland, based on the animated film "An American Tail" is well-themed and lots of fun. It's an interactive playground with lots of climbing and sliding for little ones and a fun water slide.
Perhaps the best thing in KidZone is Curious George Goes to Town which has an absolutely soaking central core. Behind that is the Ball Factory, where kids can have lotsa fun shooting soft foam balls; that may not be suitable for kids as young as yours however, but who knows?
KidZone also has Barney appearing live! If your kids are fans they'll enjoy this one. There's a tame little rollercoaster and the Animal Actors Stage (WDW has a similar attraction, so if they liked that, they'll like this).
Then there's Nickelodeon Studio where you might get to sit in the audience for a live taping of Slime Time Live. The studio tour ends with Game lab, where they play some fun games pitting kids against grownups and one kid is selected to be slimed.
Over at IOA, they have Seuss Landing, devoted to the characters of Dr. Seuss. So if your kids love Seuss, they'll love this. There is a play area called If I Ran The Zoo for kids your age and a beautiful Seussian carousel.
Over in Toon Lagoon is Me Ship, The Olive another interactive play attraction. And Jurassic Park has Camp Jurassic, yet another interactive play area that's probably better suited to slightly older kids. Your little ones might also like Triceratops Encounter, unless a really big dinosaur would scare them. It's very gentle and to a six-year-old's eyes and imagination probably looks totally real.
On the whole (and with the exception of Seuss Landing, which is truly special) I'd say there is probably more to keep younger kids busy at USF than at IOA. I didn't mention the Hanna-Barbera ride. It's a pretty aggressive simulator but it has stationery seats for little ones who would probably enjoy the cartoon view out the front window of Yogi's spaceship. I also see lots of small kids on Jaws, Earthquake, Twister, and Kongfrontation, but if your kids scare easily, these rides might be out.
Hope this helped.
Kelly Monaghan
Author of "Universal Studios Escape: The Ultimate Guide to the Ultimate Theme Park Adventure" and
"The Other Orlando: What To Do When You've Done Disney & Universal"â¡