He's 4 years old and I agree with you about teaching respect. I realize though that at that age, the liklihood of him to a) remember and b) use self-control to not do something he isn't supposed to do is not very good so in my mind, the best way to avoid an unfortunate situation is to eliminate the possibility altogether. In this case, it is just as easy (and probably cheaper) to put in a check valve and use a rubber stopper than to install valves especially since I am using flexible hose.

That's very sad about the kids that drowned.