Chinuch horaah of the day: [Regarding publications made for children:] In addition to boys and girls being separate one from the other... it should be emphasized and stand out in the clothes they are wearing. There [should be] a distinct difference in all of their garments. Different garments for the boy and different garments for the girl.
Also with the hat this should be emphasized- a special hat for the boy and a special hat for the girl...
It should stand out in the picture of the boy that his tzitzis are visible and out.
In the picture of the girl- her clothes should be suitable, in the ultimate tsnius- sleeves that are as they should be, long sleeves etc. The matter of tsnius according to most poskim begins when a girl is three years old and a day.
(Sichos Kodesh, 5741, vol.2, p.418)