Does a son who dies before his father, or a daughter who dies before him, inherit? And do their children inherit? And is the deceased's will valid in leaving his apartment to his children "for the children to meet and bond in it," knowing that the heirs are two sons, a grandson, and a full brother?

The will to leave the apartment for the children to meet is not binding, and it is not considered a waqf (endowment) unless there is an explicit statement or an implied one with the intention of waqf. Otherwise, the apartment is to be divided among the heirs according to Sharia. The son who died before his father does not inherit from him, and his children do not inherit from their grandfather due to the presence of the deceased's direct children. The daughter who died before her father does not inherit, and her children do not inherit because they are from the kin (dhawu al-arham). If the deceased has no heirs other than those mentioned, then his inheritance goes to his two sons equally, and there is nothing for the grandson or the full brother, as they are blocked by the son.