I went whitewater rafting in Colorado when I went to Young Life camp. It was so much fun, but it was a short trip on the Arkansas River, which really wasn't that rough.
I don't think this white water will be that rough. The original trip I was looking at was a 1/2 day trip, but the kids had to be 12 or over and my nephew won't quite be 12, so the trip I picked is now a 2 day trip on the river, but under 12 is allowed. I imagine it will be fun, but not too rough. My overall plan is in 2 years I'd like to take 3 of the boys on a 5 day white water rafting trip through the Grand Canyon and then hike back up it. They should all be old enough by then and it should be a good time.