Universal and IOA are not brilliant for a toddler, there are some kiddie things, but for the most part you will have to baby swap - they both offer baby swap facilities which are very good (for the most part nicer than those at WDW).
At IOA there is a Dr Suess area which has a carosuel, a Dumbo style ride, the Cat in the Hat (which my 3 year old HATED) and a really nice water play area called if I ran the Zoo, all of these would be great for the 2 year old (except possibly the Cat in the Hat, it is dark, jerky and uses strobe lights, I would suggest riding it yourself before you decide).
You can also get lunch at Circus McGurcus and the characters do a little show while you eat.
Other than that, she played on a boat in one area (Near Popeye's Bilge Barges), and that was about it. She didn't like the Jurassic Park play area at all, because there were lots of dinosaur noises when you stepped in the wrong place. There is an inside area called the Discovery Centre which was ok, not very exciting but not scary.
At Universal, there is Barney - if your 2 year old likes Barney, then you really can't go wrong! We spent a lot of time with Barney - there is a nice play area outside the Barney show which she really enjoyed, if you go right after a show finishes then it will be packed, but if you go at a different time then it will be very quiet and you should have the place to yourself.
ET is a fairly tame ride, which your 2 year old may like, but it's very dark, so it depends on the child. My dd also liked watching the Blues Brothers show, but that was about it for Universal - we did the baby swap on a few things, but we didn't spend a great deal of time there as there wasn't a lot for her to see or do.
Seaworld is an excellent park and will probably appeal to both children. It is my dd's favorite park by far, she would probably pick it over MK if asked.
You'll get loads of info on the Disney parks, so I'll leave it at that for now
Bev