Kids are kids, and whilst they may have not yet developed a sense of sexuality and desires they may still wish to look attractive and pretty(sexy is not really an adjective here,since they have no real intention of looking good in order to attract anyone in a sexual way). It's up to the parents/guardians then to teach them the importance of inner beauty and good character along with the beautifying of one's outer appearance.
My daughter is seven years old and she just loves to look all pretty and dressed up BUT she is also aware that as a Muslimah once she grows up she will need to adorn the outfit of a Muslim woman. Mind you I hardly had to teach her that, only leading by being an example.
I do however instill in her the importance of being humble and kindhearted, and to shun arrogance, pride, discrimination etc. This is important.
And whilst I encourage her to look her best and take care of her health and outer appearance, I also tell her that beauty maybe a valuable asset but it is far more important to be a good human being and never look down on others. This I feel is essential duty of any parent.