I did that so many times as a kid! If we were camping near water, I was there. I learned a lot about rivers, streams, ponds, lakes and ***oceans*** in the process.
I remember getting to stream morphology in a physical geology class. It was all so common sense to me, but a number of my classmates couldn't grasp the principles of water running downhill. It dawned on me that they had never wandered a stream bank and played on the rocks and logs. During one winter storm, the Skykomish River changed course by 1/4 mile in one night. It was awesome to see the results; fortunately, it just missed the village of Index.
Getting his feet wet will give him a love of nature--- and maybe make him a little smarter too :)
Good idea on the moss!