My son first went to the waterparks at the age of 2. He absolutely loved Blizzard Beach. There are two kids areas, one for the very young set, and one for 8-12 or so. Aaron loved the toddler area (Tike's Peak) and some of the stuff in the pre-teen area (Ski Patrol Training, I think) even at the age of 2. Some of the kids slides allow the child to ride on a tube alone -- mom and dad were scared; Aaron just said "Again!". He also absolutely loved Teamboat Springs, the family raft ride. It was, I think, he favorite thing in all of WDW. With a lifevest, he also loved floating in the wave pool. And the chairlift is great for a kid with little legs who gets tired walking up the mountain.
Since then, we've returned to BB 3 times (he's now 4 1/2). We've added to our list of rides Runoff Rapids, which has double tubes where Aaron can ride with one of us, as well as the two matt slides (names escape me), and everything in the pre-teen section except a couple of things. He loves riding everything.
On each trip, we buy a pic of the three of us taken at the end of Teamboat Springs, so we can watch him grow with the same happy smile on his face in each picture.