We took each of our kids when they were about 1 year old. They loved it because there is so much sensory stimulation. A lot of the things that scare them once they are older don't bother them when they are younger. The first time I went back to the parks as a parent I went out and got the book Walt Disney World with Kids by Kim Wright Wiley and I've used it ever since. Given that I was already very familiar with WDW, I found her suggestions spot on.
As far as parks go, my 4 year old would probably spend his entire Disney vacation at the playground in Dinoland in the Animal Kingdom if we would let him. From other parents of similar aged kids that I've talked to, the various playgrounds such as Pooh's playground in the Magic Kingdom or the Honey-I-Shrunk-the-Kids playground at MGM are tops with the kids. I've found that it's a nice break for me. They only have one entrance/exit point so I don't have to worry about them getting out. As far as rides go, the Magic Kingdom is definitely the highest density of small kids rides. Remember, if your 2 year old or even your 5 year old can't or doesn't want to ride you can use the "baby swap" system.
Finally, the things that I think will make or break your trip is more "how" you do the park than which park you do. You'll see family after family on a death march trying to make sure they get their money's worth and see everything. The fact of the matter is you can't see everything anyway. Our philosophy has always been to do Disney at a pace the kids can comfortably enjoy. That means we see what see. The first time we went back to Disney with kids, I think we only went on 6 or 7 rides. It was one of the best days I ever had!
A typical trip with our kids means we get there early, leave in the afternoon when it's hot so they can take a nap or cool off in the pool and then go back in the early evening. By the way, the crowds are lowest early in the morning and tend to peak in the afternoon. It also means we don't go to the parks every day. We usually go every other day. This gives the kids (and us) a chance to rest so they actually enjoy the park. This may seem crazy, but while most people's kids are screaming and crying we are enjoying our happy, rested kids. That'll probably change as our kids get older, but while they are young, that's the way we've done it and it's worked great for us. Our mantra is "we can always see the _______ the next time we come." Knowing that this isn't a once in a lifetime trip changes our outlook.
Whew! Excuse the long post. Hope that helps.
