Family of 4 (2 kids under 16) in 2020 taking standard deduction, no childcare expenses, no tax deferred savings.

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.