How is a sum of money that came to the deceased as a gift after his death divided? Is it divided among the heirs according to the Sharia inheritance law, or is it distributed equally among the children?

If the gift reached the deceased during his lifetime, and he accepted and took possession of it, then it is part of his wealth. However, if it reached him after his death, and he was not aware of it or did not accept it during his lifetime, then the gift is returned to the giver; because a gift is only owned through acceptance and possession. The giver can then bestow it upon whomever he wishes among the deceased's heirs.