Hi mnlovesdisney,
I took my 3 and 7 year old to both parks. They loved different aspects of both! If you decide to only go to Universal this is what my 7 year old was liked.
1. The Funtastic World of Hanna Barbera
2.Nick Studios
3.Twister
4.Konfrontation
5.Terminator 2 (has been discussed it may be scary for some kids, my son liked it)
6.Woody Woodpeckers Kidzone (Everything Here)
especially Curious George Town and woodys coaster
7.ET
8.M.I.B (this was his favorite at Universal)
9Back to the Future
If you get the opportunity I think your kids are old enough to enjoy many things at IOA as well.My kids liked it better than Universal.
Enjoy your trip!