I want to make a quick plug for early, basic disclosure. I tend to think that the more casually you talk about it, the better--and being able to talk about it casually when they are older starts by being able to talk about it casually when they are younger. You don't have to dump the whole thing on them, but IMHO a child should at least know the very basic facts of life as soon as they are interested in learning about other basic bodily functions. It is a normal, natural function, and tiptoeing around it or hiding it doesn't preserve innocence, but does introduce the concept of shame once they figure out that you've explained everything else but that.
