Praise be to Allaah. 

It is not permissible for a man to do ghusl for his mother, or for a mother to do ghusl for her son. Similarly, it is not permissible for a man to do ghusl for his daughter. A man cannot do ghusl for a woman, even if she is one of his mahrams. (The only exception is that) a wife is permitted to do ghusl for her husband, and a husband is permitted to do ghusl for his wife. Apart from that, a man may only be washed by other men, and a woman may only be washed by other women.

  If a male did not reach the age of seven, then it is permissible for a woman to wash him, and if a girl did not reach the age of seven, it is permissible for a man to wash her. But if the boy or girl was over the age of seven, then men should do ghusl for the boy and women should do ghusl for the girl. The point is that men are not allowed to do ghusl for women and women are not allowed to do ghusl for men, except in the case of husband and wife. And Allaah knows best.