Apparently I'm in the minority on this one. 5 years ago, we took our then 3 1/2 year old on it (with a life vest on) and was SHOCKED. He did fine, BUT - he was splashed in his face for most of the ride and did not like that. He rode it with his hands covering his head and I rode it hanging on to him. The ride is much scarier when you're not holding on

) and afraid for your child. He came off of it just fine and wasn't crying or anything, it just was "nothing" to us.
However, saying all of that. We took my other son when he was 3 1/2 and he loved it. But, we knew what to expect.
I say - expect that it might be a little "rough", have a life vest on him, and have fun!!!!