david23 - I disagree w/ your post.....
Apptitude is what comes to mind when I read your post. I think some kids fit what you descibe in your post, but not all. Parents should certainly be aware that not all kids are ready to start running rivers young. Parents should also be able to recognize when kids get hungry, cold, etc.
By recognizing those signs and ensuring that the kids are comfortable and needs met, most young kids can have great experiences on the water. But, it's also an experience that I personally would not entrust to someone else, not until my son is older and has cut his teeth sufficiently.
My boys (8, 5, 3) have been around the water since they were born and really love the water. I've taken the boys rafting and as a passenger in an inflatable kayak as early as age 2 (class II/III) and they've done great.
I picked up a Fun1 when my oldest was five. My wife was very skeptical of the purchase until she saw him paddle around the pool, and she was truely amazed. With the correct sized gear, young kids do have and can develop the motor skills necessary for paddling.
I took the oldest down his first run (class II) when he was 7 and he did great. He doesn't roll yet - that will come when he's ready and wants to, but he does very well paddling and with wet exits. He's picked up on the skills over the years, playing in the pool, the lake, the deep pools on the river in late summer, runs here and there with me, etc. He sat out most of last summer w/ a broken arm, and is looking forward to many runs down Filter Plant this summer.
My five year old is getting to the point where he likes to paddle around by himself. He previously, as my 3 year old currently does - preferred to be pushed around the pool, ride on my lap, float behind my boat as I paddled, or use the kayak as a springboard/launching point into the pool.
I don't push the boys to kayak - they'll be ahead of my skill wise too soon anyway. The most important thing about kayaking is that they are 1. safe and 2. have fun. I carefully gauge to ensure success in both apects, but if safety is in question or they're not having fun, no problem, the run/play is done and saved for another day.
Anyway, my 2 cents shaped by experience with my boys, cousins, nephews and other kids I've boated with.