Um... I'd be quite concerned - not so much about the experimentation, because I understand that to be quite normal, but the knowledge of 'what goes on' seemed somewhat too specific for me to be comfortable with it... I'd be very worried about where they got the info from - I think that's way too much to have been learned just from accidentally walking past a tv showing porn.
That was my first thought. I have a feeling there is a little more going on and i think you should keep an eye on the boy next door, what he knows isn't normal not at his age. If something similar happens again i would pull his mother aside and say what the hell?? i am saying this because something bad happened to my sister when she was a kid and this just shouts that something isnt right.