When three generations live in the same household, the rules for who can claim whom as a dependent get very complicated. It's not clear whether you can legitimately claim your grandson as a dependent, since he is probably a "qualifying child" of your daughter. Before you go to great lengths to get his Social Security number, you should determine whether you would actually be able to claim him. You may want to consult a local tax professional, who can go over all the details with you, or see "Qualifying Child of More Than One Person" in IRS Publication 501, starting on page 15 of the 2024 edition.
