Q. Is it sunnah for girls to have shirt length till midcalf and without slits and what is the virtue of keeping a veil on the head for girls?
A. For women and girls who have reached the age of puberty (baligh), it is compulsory for them to cover the entire body, except the face and hands upto the wrists, which, according to some reliable scholars, are allowed to be exposed. This means that the head must be covered as you have asked.
According to the teachings of the Holy Quran, a woman should cover her entire body with a loose sheet, garment, or gown. This should be worn over the clothing which she normally wears, when she is coming out from the house for any need/necessity. This is the manner in which the wives of the Prophet (SAS) dressed, and the Sahabiyaat (female companions) also dressed in the same manner. Hence, this is what would be considered Sunnah for them.
As for that which is the minimum requirement for women, it is that they do not wear tight fitted and thin clothing which reveals the body or reveals the shape of the body. The upper garment must not be so short that the posterior and thighs etc. become visible. It must also not cling on to the upper body to the extent that the physique of the body becomes revealed. If the woman is not wearing a loose gown, garment or sheet to cover herself, then it is essential upon her to wear the khimar (head piece) in such a manner that the ends fall on the chest/bosom, thus covering the entire chest so that the shape of the bosom does not become visible.
The lower garment also, must not reveal the shape, size, and structure of the lower body.
In short, a woman is required to wear such loose garment that her physique is not seen and known to anyone.
Based on this explaination, there will be no harm for girls to wear Qamees (a long shirt) which reaches to the middle of the calves and do not have slits in them.
And Allah knows best.
Mufti Waseem Khan