21st Century Worker Tax Cut Act - Amends the Internal Revenue Code to: (1) allow married taxpayers with at least one qualifying child under the age of 12 an income-based tax deduction equal to 20% of the lesser of $60,000 or the earned income of the spouse with the lower amount of earned income; (2) increase the amount of the earned income tax credit for taxpayers with no qualifying children; (3) increase the penalty for tax return preparers who fail to comply with due diligence requirements for determining eligibility for the earned income tax credit; (4) extend the limitation on the tax deduction for employee remuneration to current or former officers, directors, or employees of a publicly-held corporation; and (5) include in subpart F income (income of controlled foreign corporations the tax on which may not be deferred) low-taxed cross-border income (gross income of a controlled foreign corporation except income that was derived in the home country of such corporation or was subject to an effective rate of income tax imposed by a foreign country in excess of 15%).