I could say almost anywhere there onsite. Margharitavilles is one of my favorites in CityWalk and is pretty kid friendly. Enchanted Oak in IOA is great, but so is Mythos, but that is more of a sit down, nice restaurant, not counter type service. Well I should say Margaritavilles isn't either.
I guess counter service is going to be Enchanted Oak and the restaurant at the Jurassic Park Discovery center (as far as the better counter service places) Your kids will probably love the Circus McGurkus Cafe in Suess Landing though. Food isn't quite as good as some of the other spots, but still pretty good, atmosphere is really neat there.
Also the Monster Cafe at the main Universal Park is pretty cool.
Universal has a lot more sit down menu type nice restaurants than your typical theme park, including the ones at the hotels and the ones at CityWalk.
If things get crowded you can also head over to Jakes at the Royal Pacific, great food (sit down menu though) but kid friendly (kids menu). But very good food here and usually no wait.