Praise be to Allah.
The child – male or female – needs to know about this ruling when he or she approaches the age of puberty and when you think that this hair, which has been made one of the signs of puberty, and which we have been commanded to remove for the sake of tahaarah (purity), cleanliness and good health, has started to grow. You could introduce this idea gradually in the context of other points and ideas that the child needs to know and be reminded of as he or she approaches the age of puberty, such as the meaning of takleef (accountability), the rights of Allaah over a person, the duties of the person who has reached the age of accountability, the fact that his or her evil deeds will be recorded from puberty onwards, and the rules of ghusl for janaabah (bathing for impurity), etc. If the parents find it embarrassing to talk about this subject directly with the child, they can give him or her a book or pamphlet about the rules that have to do with puberty. And Allaah is the source of strength.