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The Child Tax Credit reverted back to the previous amounts (up to $2,000 for each child under age 17) and is no longer fully refundable. Only $1,500 of this is refundable and you will need earned income of at least $2,500 for the refundable part. If you don't have any taxes due and not enough earned income, you might not get the full Child Tax Credit for each child.
Please see What is the Child Tax Credit?, What is the Additional Child Tax Credit?, and What are the qualifications for the Earned Income Credit for additional information.