Hi - its ole butt head again...
I don't have an opinion on when is appropriate to tell the child (except maybe when you feel that its the right time - think it would depend on whether you think the child's is old/mature enough to understand).
Anyway - the reason I butted in is that I remember seeing a book in my clinic called, "Sometimes it takes three to make a baby" or something along those lines. I think it is designed to help explain to children.
Thought I'd mention it!
cheers
BlueSky