All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
It is obligatory to wash the girl, if it is possible. If you fear that her body will be torn, you must perform Tayammum (dry ablution) for her; someone hits the ground with his hands and then wipes over her face and hands. After that she is to be shrouded and then you offer the funeral prayer on her.
In his book Al-Mughni, Ibn Qudaamah, may Allaah have upon him, said: “… and the one over whom a building fell, the burned and the drowned; if it is possible to wash him, then he must be washed. If there is fear that washing might tear the body, water should be poured over him without touching him. If it is feared that water would tear the body, he must not be washed and Tayammum should be made for him if it is possible, like the living who is harmed by water. If it is not possible to wash him because of the lack of water, Tayammum should be made for him. If it is not possible to wash all his body, they have to wash what is possible to wash and perform Tayammum for the other parts, like a living individual.”
Imaam An-Nawawi said: “If it is difficult to wash the dead for the lack of water or because he is burnt so that if they washed him his body will tear, they must not wash him. Rather, they have to perform Tayammum for him. It is obligatory to perform Tayammum since it is a cleaning process with no relevance to removing the impurities. Hence, if it is impossible to use water, it will be incumbent to perform Tayammum.”
As for the question: who is to wash her if she was unmarried?
The answer is that women should wash her, and the priority is for her Mahram-women, if not then the non- Mahrams of women, if not then the foreign women. If there are no women (able to wash her) or there is a fear that they may scream, the closest men of her Mahrams should wash her. They may do that in the presence of women. However, If there are no Mahrams whether males or females the vast majority of the scholars choose to perform Tayammum for her. Furthermore, there is another scholarly opinion indicating covering her with a sheet then washing her from under it whilst the one who washes her wears gloves or the like and lowers his gaze as much as possible.
Allaah Knows best.