I think it depends on your family and what they like to do. We go and our DS is 7 and our DD 2. Universal is two parks; Universal Islands of Adventure (IOA) and Universal Studios (US) and their shopping area (Citywalk)
My DS (7) has been going to Universal for a while. He loves roller coasters etc and is tall (48") so has been able to do enough of the rides for him to enjoy. He loved the Mummy ride in US. Most of the rides / shows at US are OK for all ages - DD went on jaws when she was 5 weeks old!
IOA has the Dr Seuss area which has several rides just for kids, additionally they have a unicorn roller coaster (36" up) do my DD (2) can ride it - if she wanted to

. There is also a pteranadon (sp?) flyers which can be done at 36" tall.
For the older kids, Spiderman is great - but can be scary. Two water rides (Blutos & Popeyes) are great - but you WILL get soaked. There are more, but I can't remember them all now.
US has more shows and is more younger crowd friendly, but we as a family can enjoy both parks. DH gets to have his roller coaster fix at IOA. DD loves Barney at US DS doesn't care as long as he gets to ride the coasters.
Both parks have play areas for kids to climb and get wet in.
My favorite ride is Men in Black at US - similar concept to Buzz Lightyear at MK
New this year - blue man group have a stage at Citiwalk - they are awesome.
Check out Universal's website for good deals on passes. You need at least 2 days to do Universal properly.
Oh. Universal also do a baby swap thing should you need/want it.
We also like Seaworld - great shows, neat animals and I believe that they have doubled the number of rides in the kiddie area (Shamu's Happy Harbor). Again, lots of places for the kids to get wet. Seaworld is becoming a 1.5 - 2 day trip to experience fully.
Hope that helps.
--Kay