The $3000 proposed child tax credit you may have heard of is not law yet and will not be anytime soon.
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.
