According to the Shulchan Aruch
when a girl reaches the age of three years and a day she should already conduct herself in a manner of tznius
. Surely so this applies to a girl 10 years and older....
But not to be oppressive about this ... and to speak in a pleasant manner.
Excerpt from a letter of the Rebbe, Likkutei Sichos, Vol. XVIII, p. 448