The Hijab of a Young Girl
Question

                What is the ruling concerning the
      young girls who have not reached the age of puberty? Is it allowed for them to go out
      without covering themselves? Can they pray without wearing a head covering?
Praise be to Allah.It is a must that their guardians bring them up and teach
    them the manners of Islam. They should tell them not
to go outside unless their bodies are covered. This is in order to avoid
    any temptation and to get them used to the virtuous
manners so that they will not be a source of spreading evil. They should
    be ordered to pray with head coverings. If they
pray without it, their prayers are sound. This is because the Prophet
    (peace be upon him) said,
"Allah does not accept the prayer of a female who has reached the age
    of puberty except if she is wearing a head
covering (khimaar)." 
This was recorded by al-Tirmidhi, Ahmad, Abu Dawud and ibn Majah. Sahih
    aljam'e 2/1280