The child tax credit is only paid against income tax that you owe. If you don’t owe income tax because your income is low, you won’t receive any credit even though you qualify. There is a secondary version of the credit, called the “additional child tax credit” which can be paid even if you don’t owe income tax, but it is based on your income earned from working, and does not take into account alimony, a pension, investments, or other taxable income. Even if your entire $20,000 of income is earned from working, it is likely that you have reached the maximum amount of additional child tax credit you can receive.
