On reading this thread, I had to make a comment.
Marina, about your friend's two young girls - I would not be happy with my daughter (9) doing cartwheels around the garden naked (although she is quite developed for her age and if she wasn't so developed, may be I wouldn't feel so upset at this).
I hate the thought that my DD1 is losing her innocence and I have to tell her to cover up and not sit with her legs wide open, but I feel that I wouldn't want her to be in any situation (or get her into any situation) that could be potentially dangerous.
Another comment:
I love seeing children naked on a beach - so at nature and natural, but I do feel uncomfortable when I see older children. Not uncomfortable for myself but the older people that are seeing them, dirty old men, etc. I know that not everyone is a potential paedophile, but I remember an occasion when I was at my local park with my children, and an old man was sitting on a bench in the kiddies play area but had no children with him - it just made me feel a bit uncomfortable.
I also remember an incident from when I was younger (about 11 or 12 - a long time ago!). I was quite naive and I was in the park playing with my group of friends from school and a man asked to take photographs of us doing handstands. This we quite happily did and it is only now that I think it was all a bit deviant.