The definition of the ‘awrah of children, male and female, is something for which there is no clear evidence in the Qur’aan and Sunnah. The view of most of the fuqaha’ is that when a girl reaches the age where she may be desired by those of a sound nature, then her ‘awrah is like that of an adult woman. But if she has not reached that age, then it is permissible to make concessions about her hijab until she does reach that age. 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said: There is no ruling on the ‘awrah of a very young girl, and she does not have to cover her face, neck, hands and feet, and a small girl should not be obliged to do that. But when a girl reaches an age when men could be attracted to her and desire her, then she should observe hijab so as to ward off fitnah and evil. This varies from one woman to another; some of them grow quickly and start looking attractive and others do not.  

With regard to wearing skimpy clothes, it is better not to let young girls get used to such clothes. It is important for girls to grow up loving virtue, until they get used to it. Hence Islam gives children a preparatory stage in which they can get used to praying and does not make it obligatory all of a sudden, because it takes time to get used to it and learn it. When a girl is nine years old, she should start learning and getting used to what will be required of her, before she reaches puberty. It does not make any sense to make the difference between covering the ‘awrah and not doing so the night when the girl gets her period and reaches puberty. That should not happen. 

Shaykh Ibn ‘Uthaymeen said: I think that people should not dress their daughters in these clothes when they are young because if the girl gets used to them she will continue to wear them and take the matter lightly. But if she gets used to modesty from an early age, she will remain modest when she grows up. What I advise my Muslim sisters to do is to forsake the clothing of foreigners and enemies of Islam, and make their daughters get used to wearing concealing clothes and to modesty, for it is part of faith.

 From Fataawa al-Shaykh Ibn ‘Uthaymeen in al-Da’wah magazine, issue no. 1709, p. 35. 

It is well known that there is no ruling on the ‘awrah of a child who is younger than seven, but there can be no doubt that letting children get used to short and skimpy clothes will make them careless about uncovering their ‘awrah in the future, and a person may not be bothered about uncovering his thighs because he used to uncover them when he was little. 

So we should prevent children from wearing such clothes even when they are small.