I have three kids. Sex has never been an issue that we felt was embarrassing. My kids have seen me and my husband kiss and say "ewww, stop that, we hate it when you do that!" And my husband shot back, "If we didn't like doing stuff like this, none of you would have been born!"
Then my six year old little girl said, "just don't be opening your mouth when you do it okay?"
Gee, we had no idea we were so obvious. Oh anyway, they all know how it works and have for a long time. Ever since we used to have two dogs that...well, you know how dogs get "tied" when they make love. Kids got a good shot of that ya know.:) Then we had puppies not so long after.