Fair enough. I don't exactly see it as "hiding" if you go, "It's something that happens to you when you're older, we'll talk about it more when you are a little older" and holding off until they're six or seven. I can see why you would feel that way, though, and, yeah, it's definitely problematic. I feel like there needs to be a balance struck between "telling them the whole truth, and nothing but" and "fuck no I'm not going to tell you about puberty til you're 13 and come to me crying because you fear you're going to bleed to death". Personally, I feel like that balance is struck by telling them, "It's something that happens as part of growing up. Everyone develops hair there" and leaving it at that—but, obviously, you may not feel the same way.

tl;dr, you make a really good point and I don't feel like hiding it is a good solution either, I just don't feel like full disclosure is necessarily a good thing.