That's sort of a big question, but as a Universal fanboy, I'll do my best to answer it as informatively and concisely as possible.
Both parks are DEFINITELY worth going to, especially considering that the price difference between a one and two park ticket is not outrageous. We have an amazing time at both Universal and IOA, so definitely check it out.
As far as dining goes, the options are not what they are in Disney. So don't go in there thinking you're going to find a Cali Grill on property. The options at Universal/IOA are more like what you'd imagine from a theme park. That being said, Mythos is a fantastic place (and has been named the best theme park dining a bunch of times). THe Three Broomsticks is also a pretty awesome place, and a must if you or your kids are HP fans.
In addition, the Butterbeer and Frozen Butterbeer are the maybe the best things in the entire world.
As for the must dos. The Wizarding world is an experience that you wont soon forget. Even if the younger kids are a bit small for the rides there. The atmosphere is truly magical.
If the older kids are tall enough for the Spiderman ride, I strongly suggest it. We love Twister and Cat in the Hat; which is open for the entire family without restriction. Shrek and Despicable Me are amazing, and we love the makeup show.
Posidien's Fury is fantastic and the Dr. Suess storytime is fun for smaller kids too. Also, Univeral puts on a surpringly good parade.
Keep in mind these are only my opinions. SOme people might disagree with them, but I really think your family will enjoy their time in the parks.
Hope this helps.
Have a great vacation. I'm totally jealous reading through your plans.