The Additional Child Tax Credit isn't a credit you get for additional children; rather, it's the leftover portion of the Child Tax Credit (up to $1,400) that you're eligible to receive if the Child Tax Credit wipes out your tax liability for the year.
TurboTax automatically calculates and applies the additional child tax credit to your income tax return. It is reported on line 17b. You can see the calculation on Schedule 8812.
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