My 5 year old boy who is a little on the nervous side rode Spiderman once. He liked it, but didn't want a second time the next day. He is 44 inches tall. He loved all the rides. He wouldn't go on Kongfrontation because 10 year old sis told him he could get eaten. She learned to keep her mouth shut!
We also did not take him to Twister because we get enough of that in the mid-west.
His favorite by far wasMen inBlack, Alien Attack, and over the 3 days we were at both parks, we rode 15 times.
My 10 year old did all of the rides, but still lots to do if the 9 year old is not a coaster person.
Have your kids wear their swim suits. There are tons of opportunities at Universal/IOA to get soaked, (lot more that WDW)Then they will dry off real quick, enough to get soaked again, again.