My DS has a habit of absent-mindedly playing with himself when he is watching tv or in the bath. It does make me uncomfortable, because it is more than a little disconcerting to see your 6 year old with an erection. I just gently tell him to stop and that it is not nice to do that in front of people - in private is ok. My 2 year old DD is fascinated with her vagina atm, because we have started toilet training so it's not covered by a nappy all the time - she doesn't really 'play' with it though, it's more of an exploration thing - like she just wants to have a look and see what it is.