The new (expanded) Child tax credit ($3000; $3600 for kids under 6) is effective for 2021.
Getting the expanded child tax credit has nothing to do with filing your 2020 tax return. The additional money will not be part of your 2020 tax refund.
The 2020 child tax credit (CTC) is limited to your tax liability. The CTC is a non-refundable credit and can only reduce your income tax to 0, It can not help you beyond eliminating your tax liability. But, if you have more than $2500 of earned income, some or all of it is usually given back to you thru the "Additional Child tax credit". That is, part of the CTC may be on line 28 of form 1040 (2020 version) instead of line 19 (lines 18b and 13a in 2019). The ACTC is calculated on form 8812 and is basically 15% of your earned income over $2500. The ACTC is a maximum of $1400 per child (not $2000).
