My family LOVES the Disney water parks. We always have a spectacular time there. Both Blizzard Beach and Typhoon Lagoon are scheduled to be open in April. In fact, at least one of the two Disney water parks is open throughout the year, even in the dead of winter. The average daytime high at WDW in April is in the low 80s -- perfect weather for a water park, in my opinion. There are dedicated areas at both water parks for younger children, with tiny water slides, mini raft rides, and lots of play fountains. To take a look at this, you can click
HERE, and look for a video I made called "Water Park Fun" (currently it's on page two of the videos). You'll see two of my daughters playing in the kiddie area at Typhoon Lagoon.
That being said, I'm not sure that I would make a special trip to the water parks for a three-year-old. My experience is that while little kids do have a great time at the water parks, they are probably equally happy at the hotel pool. (And you don't have to pay an admission fee to enjoy that.) If you or your husband wants to check out the water parks and enjoy the slides there, by all means take your three-year-old with you and have a great time, but I wouldn't go just for entertainment for the child.