Sorry super long, but there is a lot going on here: sexual children, sexualising children (these are quite different things), sexual clothing and sexy dressing in women, judging clothing, pleasing/provoking the male gaze, sexual empowerment, sluttiness, slut shaming a child, judging other people's dress, judging an 11 year old's dress, judging other people's standards and parenting. Some of this is about children, some it really isn't. And I think we tend to start the conversations about children being teens/looking adult/sexual/whatever and not like children far far too early. I am currently working on the premise that they are children till about 21!
One thing to remember is that kids are always are trying stuff out that they don't fully understand; neither the celebration of their bodies (which is a very adult approach to adult perceived problems, kids just are till we or society tells them they are something, be it nice, or fat, or sporty or whatever), nor do they really "get" the sexual implications that certain clothes and combinations of clothes communicate to us as grown-ups. We put those interpretations on them and reinforce behaviours by our responses (you look pretty is said to girls from the moment they emerge - so not unexpected that they might try to engender that response as they become more autonomous). So how we react, don't react or shape the interactions is key I guess (just as was when they were two and bellowing "NO!").
Just like with language and behaviour kids of all ages do need to try things out, but as parents we are there to guide them through and set limits where we think things might be inappropriate (in public, at school, in the garden, with non-family around etc etc etc). They try things out to get reactions, gauge response, try on a new way of being, be like their friends - all in order to see where they fit, what happens next, what reactions they might get. Too right that an 11 year old "doesn't understand the effect she has on men" she shouldn't either. She shouldn't be judged for that, surely? She should be guided and protected, but that is almost impossible to do when you are not her parent and not something that wider society seems particularly good at. In any case none of it is her fault or her problem even - the issue is the adults around her. The man who asks for her number (as happened in a corner store locally to one 11 yr old girl we know), her parents who think it's okay (and they may have their own well worked out reasons and logic!), the other parents who judge and categorise her, the retailers who sell padded bras to children etc, the advertisers who use 12 year olds as models for adult products....If they do have a precocious knowledge and presentation of their sexuality at 11, while that might be where they are personally in terms of their development, there is also a chance that they have been exposed to a great deal more sexy (and I would still contend that they are not capable of fully understanding their sexuality at that age, let alone adult sexual response) than you might hope for your own child.
Inappropriateness is very subjective of course, and as they approach later childhood and early teens there is a whole new territory to navigate. I think though at Year 6 (and for that matter 7 & 8) they are still very much kids and it is surely fine to say you are too young for this or that, we believe this and behave this way at home, but other people do this because they think this is ok. I am constantly surprised by how many people say "oh so and so looks much older" about children - at 11/12 I am really struggling to see it. But it gets repeated and repeated and they then slip into acting as if the child IS older. Developed 11/12 year olds have a few of the characteristics of an adult - but it is REALLY (IMHO) rare to see an 11 year old who genuinely could pass for 16 (as a nominal age for sexual maturity).
So OP - finding a way through is all there is! I am navigating too with an 11 year old. We talk about it - she is uncomfortable with what she doesn't have the language for, but is in fact the too adult appearance of some of her peers. And yet she is increasingly interested in looking "nice" - wearing a bit of make-up, even low heels for special occasions. I think this is ok. I loved to look sexy at 17 onwards (and of course this starts earlier for lots of people), but I am hoping we can get nearer that age so that she does it in full knowledge that is an expression of her sexuality and she is control.
Phew. Sorry. Thinking a lot about this too!