Haven't read all replies but here's my take:
1. My daughters, when they wear dresses at an age where they're still prone to tumbling but are out of diapers, have always worn shorts, leggings or tights under dresses while playing. Diapers, not so much. If a kid is still in diapers, therefore not potty trained, therefore probably not developmentally "there" on learning to cover their butts in public.
2. My son will be told to keep his underwear covered outside the house too, once he's old enough to understand the concept of appropriate behavior. It's just good manners to not go waving your underwear around in public.
3. Teaching kids that "where your underwear covers is private" is an important safety lesson... My mother taught it to me, and when a stranger reached out and grabbed my crotch in an alley when I was 9, pulling away and running and yelling "No" because that's *private* probably kept me a hell of a lot safer than if I hadn't been given a really clear message that the only person who got to touch that was me.
4. I would never tell anyone to "cross their legs" to hide their crotch... crossing legs is bad for circulation. No one should do it habitually, least of all the kids of someone with a clotting disorder. (My teenager inherited the same disorder from me)
5. That said, I don't think it's a bad idea to teach kids more formal manners for more formal situations. And more formal manners can include sitting with knees together, with good posture, for either gender. But it's not something to insist on away from a party table or afternoon tea with Grandma. Teaching kids this kind of manner can be done in a fun way, and it's good practice for kids to learn that some social situations call for different kinds of "good manners" than others.
6. Likewise, at home, we're far less concerned about covered underwear than say, at church, or in the grocery store. Different manners for different places.
ETA: Skimming back... oh my god. The language debate... We've always just called it a crotch. Works for either gender. Been called that for centuries. Then again, we've had girl children only in the family going back sixty years, so the whole differentiation thing doesn't usually come up much until kids are four or five. I'm sure that will change when my son is born. My niece uses "yoni", and I'm fine with that. I prefer the word to vagina, for etymological and anatomical reasons. I don't really like kids of either gender running around and yelling about their penis or vagina casually. Once they hit 5-7 years old, I have a bunch of midwifery books and "A New View of a Woman's Body", which has the most amazing detailed drawings of both male and female anatomy, with scientific names, etc.
I answer all questions about body parts, where babies come from, sex, etc. as frankly as seems appropriate given the questions being asked.
I thought pee came out of my clitoris until I was at least 8 years old, so helping kids understand what exactly is where is something I'm happy to do. I'm fine with my girls knowing what their parts are called, but for most usages, crotch or yoni is sufficient, and if we need to get more specific, we'll pull out the diagrams and get more specific.
Edited by jenrose - 9/20/11 at 3:39pm