Is it permissible to give zakat al-fitr, zakat, or sadaqa to a man's needy grandchildren from the daughter's or son's side?

It is permissible to give Zakat and obligatory charity to the daughters' children if they are eligible for Zakat, because they are not heirs. As for the sons' children, it is permissible to give them Zakat if their sustenance is not obligatory upon the Zakat payer; however, if their sustenance is obligatory upon him, then it is not permissible.