He's just turning 4 now, and loves it. He's only done flatwater so far, but comes to all the pool/lake sessions. He still can't really swim, and is not a fan of going underwater yet, but has gotten comfortable floating in his PFD. Swim lessons are in the works for this summer. One thing I do in the pool to get him more comfortable with going underwater is to hold him to my chest and wrap him up in my sprayskirt and pretend he's a baby kangaroo in my pouch. Then I'll hop like a daddy kangaroo and sink deeper with each hop til we submerge. I'm getting him comfortable with being underwater, but he thinks he's just having fun.
Despite the fact that he's still too small to properly fill out the Fun1 and be able to edge it with his knees etc, he has learned great boat control. I got a rubber ball small enough that he can pick up one-handed and I'll throw that around the pool and have him chase it. Then we'll take turns throwing it back and forth into the open cockpits of our boats as we paddle around. I think this has helped become comfortable trying to paddle with one hand and learning the stability of his boat. He also loves for me to pick him up (in his boat) over my head. He yells, "Boof!", then I throw him so that he lands flat and shoots away, turns around and paddles back for more. Kinda like playing fetch with a dog, lol.
As for moving water, I've taken him out to a very low flow play feature where I can stand upstream of him and loop about 3 feet of webbing through the bow loop of his boat and let him feel what it's like to simply surf back and forth. He's usually bored with this after 15 minutes, so then we'll just lay down in the rapid and feel the water, seeing how our different hand/body positions affect the flow. Again, he just thinks he's having fun. I mentioned to him about doing his first real river trip, and that's all he asks about now. We just need some water. I'll probably take him on Class I in my canoe with a cooler and various toys, and plan on stopping/exploring often.
There's some great advice from previous posters on this thread, and I think it all comes down to making it fun. Several times I've picked him up to go "boating" and he wanted to do something else instead, so we did. No pressure, be flexible. Let them fall in love with the river at their own pace.