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.
The money you hear about people getting for just filing a tax return claiming kids requires them to have some earned income (wages or self employment). Without earned income, they are not eligible for the "refundable" Earned Income Credit or Additional Child Tax Credit. Both credits are calculated on the amount of earned income you have. No earned income means no "refund". A small amount of earned income means a small refund. The child tax credit does not "kick in" unless you have at least $2500 of earned income.
A child can be the “qualifying child” dependent of any close relative in the household. If you live with someone else, e.g. your parents, it may be better if they claim your child.
Instead, you could allow the non-custodial parent to claim the children. Non-custodial parents are allowed to claim the child tax credit, but not the Earned income credit.
If you are a student, over age 23, and are not claimed as a dependent by someone else (e.g. your parent) you may be eligible for the up to $1000 refundable American Opportunity (tuition) Credit. That credit is not dependent on having either kids or earned income. You must be at least a half time undergraduate student. There's even a loop hole available to claim the credit, if you are on scholarship.