I went to both parks on January and purchased the FOTL ticket or express pass for 15.99/adult while I stayed on Disney property. Yes, you only get to use it once per ride, but you can always get an extra pass at the regular booths. Know, if your kids have never been there, I would definetely make sure they get to see both parks. IOA has the best ride in all of the Orlando area, Spiderman, some great rollercosters and water rides that kids your ages will enjoy a lot. We went with my nephew who is only 9 and for him (thrill lover), this was the BEST park he has ever been. He got really upset because he wasn't tall enough for Hulk. Can't wait to take him in a couple of years to enjoy that great coaster.
US doesn't have so many thrilling attractions, but it still is a great park nonetheless. The mummy, Shrek, Terminator 3, Men in Black, Jimmy Neutron ride and Back to the future (that's my order of preference) are great, fun and entertaining. The good thing about the express pass system is that you don't have to walk that much through the park to go from one ride to the other in order to get the fastpasses as you have to do in Disney. In january we got the parks a little before 11 and by 6 we were done. You can then take some time to walk around Citywalk which have great restaurants and shops. It is no DTD (the best), but it's at least something different.
Check on current offers before booking. You might find that it's better to buy a special promotion ticket than buying the whole package. When we went, for each adult ticket they have us a child ticket. I think right now they have an offer that if you buy a 2-day ticket you can go to the park for 5.