When your kid does something that borders on obscene or overtly sexual (and of course, your kid is completely in the dark about what that would mean), how do you tell them to knock it off? What if it is a bit more subtle like DD pulling her shirt down over her shoulder and then fluttering her eyes flirtaciously? What is the line of what you address and what you don't? TIA!

DD -- 5YO
DS -- 3YO
Re: How to reprimand slightly obscene gesture?
In that case I'd address the clothes part and not mention the eyes.
One of mine habitually lifts his shirt up and pushes it out with his elbows as he does it (hard to describe). Not only it is inappropriate for him to be randomly exposing himself but it's stretching out his shirts.
I'd explain that it ruins her clothes and ask her to stop using that logic.
I'd also be curious so I'd ask where she learned to do that to see if you can uncover the source. Not that you'd do anything about it but I like to know who's teaching them what at school so I can keep an eye on those relationships.
Total score: 6 pregnancies, 5 losses, 2 amazing blessings that I'm thankful for every single day.
I like to stick by "you can only do that in your room/at home/with Mommy & Daddy" rule w/o giving too many details - DD seems OK w/ that most of the time.
I also stick to literal stuff like "that ruins your shirt"