Hi James!
Thanks for your question. That's a tough one, as I took my 2 children (also 4 years apart) to both water parks around the same age as your children.
Some of their favorite experiences at each park include:
Disney's Typhoon Lagoon Water ParkCastaway Creek: Nothing is more laid-back than chilling in an inner tube on a lazy river! The coolest part is the lazy river circles around the entire park, so you can get a nice tour of all the attractions and neat stuff without having to walk from one end of the park to the other.
Bay Slides: This a body slide, but smaller in size to accommodate both children just the same. My children would have their mini-race contests on the slides, since they were twin body slides.
Ketchakiddee Creek: This one may be likely more fun for your 3-year old; mini-fountains to splash around in and such.
I should also mention that throughout the park there are plenty of beach chairs and sitting areas. So if you find one in a nice shady area, Dad may be able to enjoy some "laid-back" time of his own! :-)
Disney's Blizzard Beach Water ParkCross Country Creek: This is also a lazy river, but between this and Castaway Creek at Disney's Typhoon Lagoon Water Park, we give this lazy river the edge. There are some really cool cave areas you go under with waterfalls, so it feels more like a slow-moving expedition than your average lazy river.
Melt-Away Bay: This is a nice wave pool for all ages. Although the waves don't get incredibly high, try to stay away from the edge of the wave pool as much as possible. My daughter drifted off to the edge and when the waves picked up some she skinned her knee pretty bad, which we had to get bandaged at their medical station. The people there were incredibly friendly though.
In my estimation, you can find both to be fairly laid back and kid-oriented. However, if I had to give the edge between the two, I would probably give the edge to
Blizzard Beach. It seemed to have a slight edge in fun things to do, but what really impressed my kids was the theme throughout the park. Going through a beach that's "themed-out" in snow? How cool is that!
Thanks, and have a Splashin' Good Time!
Terry