There is no fixed amount per child. Your refundable credits are calculated based on the amount of your earned income. It consists of two parts: The Earned income Credit (EIC) is looked up in this table https://apps.irs.gov/app/vita/content/globalmedia/earned_income_credit_table_1040i.pdf. The "Additional" Child Tax Credit is calculated on form 8812, but is basically 15% of your earned income over $2500, up to $1400 per child. https://www.irs.gov/pub/irs-pdf/f1040s8.pdf .
The $2000 Child tax credit, you may have heard of includes a $600 non-refundable portion. The ~$4000 figure you may have heard about was the exemption deduction, from previous years. It was not a credit, but a deduction from income before tax is calculated. Starting in 2018, that is no longer available.
For children over 16 and other non-child dependents, there is a $500 (non refundable) other dependent credit, rather than the Child Tax Credit. Your children 17-18, and full time students 19-23 still qualify you for child based EIC.
