FICA Payroll taxes paid 50k * .0765 = 3825
Tax Burden 50k - 24800 (Standard deduction MFJ) = 25600
25600 = $2676.90 in FIT taxes owed before credits
Child tax credit $4000 EITC ~$750
Total Credits $4750
Total Federal tax burden -4750 + 3825 + 2677 = 1752/50000 = 3.5%
So not zero, but pretty darn close.