I have yet to stay on-site at Universal, but speaking just for the parks, i love them! IOA didn't have a whole lot to amuse my kids, but you're are not as young (& short LOL) as mine (5 & 2), so you won't have to deal with the same restrictions in that area.
A few suggestions:
US: Everyone will LOVE ET. There's a Woody Woodpecker coaster there for the kids. It's a bit more than Goofy's Barnstormer, according to DD (5). She went on this one about 10 times. It would be great for your 8 year old especially. The Curious George play area could take up a whole day in itself, with my bunch, but beware...you can get soaked here with no notice & when I say soaked, I mean it. Don't be dumb like me & leave your cell phone at the top of an open drawstring bag as you walk through. I actually had to pour water out of the bag when we left. Some people hate this...I personallly LOVE the water play! There's an indoor section here too (with footprint patterns to show you the water-free places to walk to get to it) where there are little soft nerf-like balls that you can shoot, vacuum, & toss around a big 2 story open building. There's also a ball drop every now & again. FUN!!!!!!! Jaws is pretty good & probably ok for your kids' ages, but the fire scenes get HOT. Back to the Future is awesome (height restricted...not sure of the minimum, but DD is close, so you should be fine there). I hear they're having cool Scooby activities here with the movie coming out, too!
IOA: Spider Man, Spider Man, Spider Man!! Dudley Do-Right is a log flume that (in my opinion) is better than Splash Mountain *ducks objects being thrown*. The name of the ride is escaping me...Bluto's Barges (?)...the rapid raft ride...KILLER!! Again, prepare to get wet. We all loved the Jurassic Park area. Men In Black was great too. I swear, I think that really WAS a live dinosaur! LOL And the coasters at IOA are second to none, especially The Hulk!
If you need any further info, just say the word. I love US/IOA & have been quite a few times, so these are just the basic highlights.