Are the grandchildren whose father died before the grandfather entitled to a share from the grandfather's endowment by the word "offspring," even though they are not among the heirs based on his will?

The term "offspring" includes the children of the son. Therefore, if your grandfather endowed a Waqf for the needy among his offspring, then you, your brothers, and your sisters are included in it, provided that you are needy. Your uncle (the supervisor of the Waqf) does not have the right to prevent you from benefiting from the Waqf, and you may refer the matter to the Sharia court if he denies you your right.