Military Families Financial Security Act - Amends title XVI (Supplemental Security Income) (SSI) of the Social Security Act, with respect to income eligibility for SSI, to provide for the treatment of cash military compensation other than basic pay as earned income, and of certain housing payments as in-kind support and maintenance. Expands SSI eligibility to disabled children who are born to or who become blind or disabled applicants for SSI benefits while living with parents who are military personnel stationed outside the United States.