Social Security Expansion Act This bill increases benefits and certain taxes related to the Social Security program. 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 student. Changes to taxes include (1) increasing the net investment income tax and making active trade or business income subject to this tax; and (2) extending payroll taxes on wages, salaries, and self-employment earnings to income above $250,000 (the maximum amount subject to the Social Security payroll tax is currently $160,200 for 2023). 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.