Universal may not be as kid-centric as Disney. But there really is plenty for little kids to enjoy at both IOA and Univ. Studios. They have more than enough kid stuff for one day.
Some of the kiddie rides that large adults can go on:
Caroseussel
Cat in the Hat
One Fish Two Fish
High in the Sky Trolley Train
E.T. ride
Woody's Nuthouse coaster
Flight of the Hippogriff
Simpsons ride
Kang & Kudos
Storm Force Accelatron
And they have the cute "Barney the dinosaur" live show for toddlers.
If rides are not your thing, there are a whole bunch of interesting play areas for little ones (Camp Jurassic, If I Ran the Zoo, Fievel's playland, Curious George Goes to Town, Barney's Backyard). I think Camp Jurassic and If I Ran the Zoo play areas are fun enough for even adults to play around in.
Plenty for little kids IMO. There is also a Hello Kitty store if you have little girls. And Despicable Me Minion Mayhem and Shrek 4-D they might like.
You can also do character breakfasts with various characters (Spider-man, Scooby Doo, the Minions, etc) if that is your thing. Character dining at the Universal hotels, you can also meet the characters in certain character zones in the parks.