I hate to break this to you....but do your children attend school? day care? sleepovers? girl scouts? even youth group? do they ride a bus to school? have they ever been allowed to watch tv? read a book? look at a magazine?
This innocence you claim your child still has probably doesn't exist. At least not to the extent you think it does. She might not know the ins and outs of the adult world, but you'd be a fool to think that she hasn't heard "bad words" or listened to songs that thinly veil adult situations (my goodness what do you think every Britney Spears song is about). By the time she was in third grade she had probably heard talk from other kids at school about sex and drugs. I didn't go to an "inner city" school, or one in a bad district. My elementary school was in Palm Bay FL. Not exactly a thrieving metropolis, but a fairly decent place to raise kids. I didn't really know how sex worked in 3rd grade, but I had heard about it. There will always be kids who know things, out of their age range, that they picked up from tv, movies, even parents and older siblings. They will talk about this in school, and in any other gatherings. Why? Because they are curious.
It might be easier to pretend that this doesn't happen, or that your daughter is the exception, but you are only fooling yourself.
I am not promoting the loss of "innocence" as you put it. Goodness knows children shouldn't be subjected to looking at websites containing, as one man put it, "professional women" or even non professionals. The internet is a scary place. While children should learn how to navigate a computer and do research for school or other activities perhaps you should not allow unsupervised internet time when you are not around.
As far as I know the DIS board has never had a disclaimer asking posters to watch what they say because 12 year olds might be reading this. 12 year olds are probably reading this, but that doesn't mean that people should not be able to talk about adult things. A lot of people don't like their children watching the news because of the violence, or discussing death and tragedy. So should no one be allowed to post news stories on the board? Or ask for sympathy when a loved one has died? People post about the things going on in their lives. Some people are dealing with infidelity by their partners. Do you suggest that those posts be discussed through PMing only to prevent your daughter from knowing that even though people have taken vows to love and honor each other before the eyes of their "god" and friends and family, that all that can be thrown away if one or more of the partners wants to add a little "spice" in their lives?
Please excuse me if my post comes across a little strong. Dream I do not mean to offend you or anyone else. These are just my opinons, and while we will not agree on everything, I hope that does not mean we can't be friends.