Based on what you're saying, 2 days are more than enough. Is it possible to stretch it out to 3 or more days without getting bored?
I've stayed at Universal for 3 nights, 4 days before and had no trouble finding stuff to do. Especially with that age group, you'll probably move through the parks at a leisurely pace - and particularly with the shorter operating hours - you'll have plenty to fill at least 3 days. If you move a little faster, you can probably hit everything you need to see in 2 days, as well.
Let me just begin with this: If you want to do Rockit, you either have to be there first thing in the morning and wait for them to open it, or be there at the very end of the day and pray it doesn't break down. They opened it a bit sooner than they should've and it's been running at a very limited capacity. So either make that your first or your last stop on one of the days - I haven't ridden it yet, so I can't comment on its actual quality... but most people who have tried it enjoyed it considerably.
Here's my list of attraction opinions:
Not to be Missed
Revenge of the Mummy
Jaws
E.T. Adventure
The Simpsons Ride
Men in Black: Alien Attack
Terminator 2:3D
The Universal Horror Make-Up Show
The Amazing Adventures of Spider-Man
Dueling Dragons
Jurassic Park River Adventure
The Incredible Hulk Coaster
Cat in the Hat
Priority B
Disaster Studios
Twister
Shrek 4D
Dudley Do Right's Ripsaw Falls
Popeye & Bluto's Bilge Rat Barges
Poseidon's Fury
The High in the Sky Seuss Trolley Train Ride
Beatlejuice's Graveyard Revue
Priority C
Jimmy Neutron's Nicktoon Blast
Doctor Doom's Fearfall
The 8th Voyage of Sinbad
Carouseussel
One Fish Two Fish Red Fish Blue Fish
Pteranadon Flyers
Storm Force
Woody Woodpecker's Nuthouse Coaster
A Day in the Park with Barney
I think I got them all in there... I didn't include Fear Factor since it's currently closed, but don't worry, it was a crappy show anywho. If you only have one day or something, then the "Not to be missed" attractions are obviously the best both parks have to offer. Priority B are still excellent, but if there are time constraints they can be overlooked. Priority C are mostly the smaller or less elaborate rides - again, most of these are still fun and enjoyable, especially for younger children. (I did not include Rockit on the lists since as I mentioned I haven't gotten a chance to go on it yet, and in addition, its operating schedule has been quite erratic.)
Seeing as you have a 4, 7, and 13 year old, plan to spend a lot of time in "Me Ship, the Olive", Camp Jurassic, If I Ran the Zoo, Fievel's Playland, and Curious George goes to Town... these are all very elaborate play areas that offer surprises even for adults. Let your kids loose here if you get tired - they'll collectively get hours of enjoyment out of these children's wonderlands. There's even a water slide over in Fievel's Playland if you're interested. As far as rides are concerned, I've always found that if the child is tall enough to ride it, they can probably handle it. The water rides aren't scary other than the drops they entail... though there a few frightening carnivorous dinosaurs on Jurassic Park you might want to be cautious of. Your 13 year old should be fine on everything, assuming he doesn't mind coasters. One thing I'd reccomend though is to tell your younger children that there will be some moments during Poseidon's Fury! where the theater is plunged into complete darkness... in addition, during the Horror Make-up Show, there's a film reel with clips from a bunch of very frightening movies. They give you ample warning when they show you this clip, but I'd reccomend covering your younger children's eyes if scary images bother them at all.
As for dining... 3 of the 4 full service restaurants in the parks have given me great experiences. Lombard's Landing and Finnegan's Bar & Grill in Universal Studios are excellent, and Mythos in Islands of Adventure has been consistently rated the best theme park restaurant world wide. Confisco's Grille I found to be meh, but it's been quite a while since I've dined there, so it may have changed. Citywalk offers a plethora of full service, as well as quick service, eateries as well, which is especially useful during this time of the year when the parks start to close earlier and it's still too early for dinner. If you need any recommendation for counter service, I always found Circus McGurkus, Cafe 4, The Burger Digs, Midway Grille, International Food & Film Festival, and the Kidzone Pizza Company to be the best counter service locations in the parks.
If you have any other questions, please do feel free to ask! :wave: