You will only be able to claim the Child Tax Credit if your child has a Social Security number. Please see this link for additional requirements that must also be met.
If you are able to get an ITIN for your child, then you might be able to claim the Credit for Other Dependents. This credit is $500 per qualifying child or relative.
If you are able to obtain either of these numbers for your child, follow the instructions in this link to amend your returns to include the applicable credit.
As far as the ITIN, you do have a few options available. SInce you filed your 2023 tax return on time, you may be able to submit the requested documents to the IRS per the letter you received. Please see this link for more details from the IRS.
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