Is it permissible to register the child of the other wife in my aunt's name, and what is your advice for their children?

Circumcision for a boy is a Sunnah, and it should be performed even if he is older. The fostering, upbringing, and care of a child are righteous deeds for which the doer is rewarded. Registering a child with a name other than that of his father and mother is not permissible except out of a recognized necessity, and on condition that it does not result in altering the reality of his lineage. Our advice to children and their parents is to raise them well and teach them the Book of Allah and the Sunnah of His Messenger.