In the case of a monthly benefit for a month prior to that in which the individual, on whose wages and self-employment income such benefit is based, dies, the waiver must be filed by such individual; and such waiver shall be irrevocable and shall constitute a waiver on behalf of himself, his wife, and his children. If such individual did not file such a waiver before he died, then in the case of a benefit for the month in which he died or any month thereafter, such waiver must be filed by his widow, if any, and by or on behalf of all his children, if any; and such waivers shall be irrevocable. Such a waiver by a child shall be filed by his legal guardian or guardians, or, in the absence thereof, by the person (or persons) who has the child in his care.