Social Security Expansion Act This bill increases benefits and certain taxes related to Old-Age, Survivors, and Disability Insurance. Changes to benefits include (1) increasing the primary insurance amount for certain beneficiaries; (2) revising the method of calculating cost-of-living adjustments; (3) establishing a new minimum benefit for certain low earners; and (4) allowing certain children of retired, deceased, or disabled workers to receive benefits until age 22 if they are a full-time students. Changes to taxes include increasing the net investment income tax for certain taxpayers and extending payroll taxes on wages, salaries, and self-employment earnings to income above $250,000. Under current law, the maximum amount subject to the Social Security payroll tax is $132,900 for 2019. The bill also combines the existing Federal Old-Age and Survivors Insurance Trust Fund and the Federal Disability Insurance Trust Fund into a single Social Security Trust Fund.