No. For one thing the increased child tax credit is not for 2020 returns. It will go into effect for 2021. Secondly, you cannot get CTC for a child who is not your own child. If your GF's children live with you and you took care of them all year, you can only get the $500 credit for other dependents for them. If you get married and become a step-parent--that changes. Or if your GF has income of her own---she can claim her children and get the child-related credits.
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