Placing the deceased’s children in the position of their father in inheritance

Question:
Honorable Shaykh, this questioner says: We have a custom where, if a man has children and is married, and then he dies while his father is still alive, his father places his children in the position of their father. This means that if the grandfather dies, these children share the inheritance in the same way their father would have, alongside their uncles, as if their father were alive. Does this practice have a basis in the Shari'ah, whether with the heirs' consent or without it?

Answer:
If the deceased’s share is one-third or less, there is no obligation. As for a person giving one-third or less of his wealth, it is not obligatory, and these [children] are not entitled to it in the presence of their uncles. He may give them one-third or less of one-third. However, if it is more than one-third, it is not permissible except with the heirs' consent after death. If they consent after death, it is valid; otherwise, the right belongs to them, and they take what exceeds one-third.