All praise be to Allaah.
In Islam, a person is not considered to be accountable until after reaching puberty. Before reaching puberty, a person is not held accountable for his actions, because the Prophet (peace and blessings of Allaah be upon him) said: “The Pen has been lifted from three: from the child until he reaches puberty, from the sleeper until he wakes up, and from the one who has lost his mind until he recovers.” (Narrated by Abu Dawood, 4402).
Based on this hadith, a girl has to put on complete hijab when she reaches the puberty.
There are three signs of puberty that males and females have in common:
1 – Nocturnal emissions (wet dreams)
2 – Growth of coarse hairs around the private parts
3 – Reaching the age of fifteen
In the case of females, there is a fourth sign, which is:
4 – Menstruation
If anyone of the signs of puberty appears in a girl, then she is obliged to do complete hijab and perform all obligatory duties and avoid all haraam.
It is however, the duty of parent or guardian of a girl to guide her towards what is right and make her get used to doing the obligatory duties and avoiding haraam things before puberty, so that she will grow up with that and it will not be too hard for her to adhere to that after she reaches puberty. This is one of the fundamentals of parenting in shareeiah.
The Prophet (peace and blessings of Allaah be upon him) said: “Train your children to pray when they are seven years old, and smack them if they do not do so when they are ten, and separate them in their beds.”
Narrated by Abu Dawood, 495; Ahmad, 2/187, from the hadeeth of ‘Amr ibn Shu’ayb, narrated from his father from his grandfather.
Something similar was mentioned in the hadeeth of Saburah ibn Ma’bad which was narrated by Abu Dawood (494) and al-Tirmidhi (407). He said, it is hasan saheeh. The hadeeth was classed as saheeh by al-Albaani in al-Irwa’, 247.
Al-Bukhaari (1960) and Muslim (91136) narrated in their saheehs from al-Rubayyi’ bint Mu’awwidh the hadeeth concerning the fast of ‘Ashoora and when it was enjoined on the Muslims. In this hadeeth it says: “And after that we used to fast it – meaning ‘Ashoora – and make our small children fast, and we would go to the mosque and make them toys out of wool, and if one of them cried for food we would give them that toy (to play with) until iftaar.”
According to a report narrated by Muslim: The toy was given to the child to distract him until they complete their fast.
Al-Nawawi said in Sharh Saheeh Muslim (8/14): This hadeeth describes training children to do acts of obedience and making them get used to doing acts of worship, but they were not accountable.
Ibn al-Qayyim said in Tuhfat al-Mawdood bi Ahkaam al-Mawlood (p. 162): “Even though the child is not accountable, his guardian is accountable and it is not permissible for him to enable him to do haraam things, so that he becomes used to them and it is difficult to wean him away from them afterwards.”
And Allaah knows best and is most wise.
So parents must teach their children since the very childhood so that they find following Islam easy and will not be drawn towards immorality and haraam.
And Allaah knows best.