It depends on your definition of "scary."
IOA has many thrill rides including rollercoasters.
US has rides that may have scary situations but aren't more than a simulator ride for most of them.
Here is a bit of description of some of the most popular rides to base your decision on. Hope this helps
IOA:
Hulk: rollercoaster. Speaks for itself.
Spiderman: For younger kids it may be a bit "scary" because with the 3-D glasses it looks like everything the villians do are at YOU. It's a dark ride and there is a fire explosion at one point (non-3-D...REAL fire). Many love this ride though.
Popeyes: Not really scary, however, I have seen a kid freak out on this before. If the water raft/boat rides are usually too rough and scary for your kids, then pass this one. The situation isn't too scary...just maybe how rough the current and drops are.
Dudley's: The big drop at the end would probably scare them. It scares ME
Jurassic Park: On a boat ride where you enter an area where you shouldn't...where the raptors are. You see a downed boat that resembles yours and the leader "freaks out." It gets really dark and is kinda creepy knowing you are "surrounded" by raptors and then you encounter a HUGE T-Rex. This would probably scare them. There is also a drop on the ride.
Dueling Dragons: Rollercoaster
One Fish, Two Fish: kiddie ride. Not scary.
Cat in the Hat: kiddie ride, not scary.
If I ran the Zoo: A little water play area for the kids. Pretty neat.
There are two shows at IOA:
Voyage of Sinbad: May be a little scary with the explosions and the evil lady. A bit loud. VERY cheesy, let me tell you.
Stunt Show: Only other show at IOA...just a bunch of people doing stunts on their bikes, skateboards, and rollerblades.
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US:
Jimmy Neutron: it's a simulator. I do not believe this ride is "scary" in the content aspect of it. Of course there are "bad guys" but since they are all Nick cartoons, and is focused towards kids, it isn't as bad as many rides are.
Shrek: Some parts will probably scare kids. It's sort of a simulator, but is more in a style of a theatre. There is Lord Farquad as a ghost, an evil stone dragon, spiders (and at one point it feels like the spiders are at your feet which may scare kids who have a phobia of spiders...). Also, the pre-show may scare kids too. The employees are really in character acting like you'll be tortured and everything. Movement wise, it isn't too bad.
Twister: Not a ride...more of a show. You stand in a room and watch a "tornado." and how it damages a town. There are loud noises and fire and may scare young kids.
Earthquake: Pre-Shows aren't scary but probably won't interest your kids. The ride itself, however, is you are on a subway and you enter a station when an 8.3 earthquake hits. From power going out, a truck and incoming subway almost colliding with your subway, fire, and lots of water. Noises are very loud too. It might be a bit scary.
Jaws: a mechanical shark "pops up" near your boat. May scare of kids...especially if the employee is REALLY in character. There are a few loud noises and some fire.
MIB: Shouldn't be scary at all. It's an interactive ride where you shoot as many aliens as possible. Though it's a bit dark and the aliens aren't as cartoon like as on Buzz Lightyear at Disney (though they don't look realistic anyway), this shouldn't be the least bit scary. Many kids love this ride.
Back to the Future: Another simulator. Very rough. Travel and almost be eaten by a dinosaur, enter a volcano, crash into things...I do not think this is all that scary. I heard of kids that really like this ride too.
ET: A kiddie-friendly ride. Dark and though seeing the police cars pull up all of a sudden may startle them. But not scare them. The flying over the city is really neat. Make sure the kids sit on the far left so they get the best view of it.
Curious George and the other Kiddie Areas: Kid-based. Kid-friendly
In addition to rides, Universal Studios has some shows.
Horror Makeup Show: I do not recommend this for kids. Kids go see it, but I can't believe parents would take them when there is a PG-13 rating. Scary monster images appear on screen some, but also some "inappropriate jokes" are told. The effects they do on the actual stage may scare kids because they don't understand what is fake. I saw a kid really freak out once. Go see this show if you ever have the chance...without the kids. It's worth seeing.
Animal Planet Live: fun show that is family-friendly. Kids would like this show a lot. Nothing scary.
Beetlejuice's Graveyard Revue: Another PG-13 rating here. Some inappropriate jokes at the beginning and some "suggested" things throughout the show, as well as an explosion or two (if I remember correctly). But other than that, should be OK. Lots of singing and dancing. Anyone else's input? Been about a year since I've seen the show.
Ghostbusters: Saved the best for last. lol. Anyway, depending on the Beetlejuice character, the beginning may have a few "suggestive" comments and whatnot. But it's only minor. It's not the least bit scary and you'll see why when you see it. Many kids like it, from what I could tell. I know of a young boy that saw the show a few times and he really liked it. He was probably between 6 and 8.
If you have any other questions, let me know. I hope this helped you a little bit.
Anyone, please make corrections if I stated something that isn't true or if I forgot to mention something that may make a ride "scary."
