You may be out of luck for this year. Here's the information from the IRS website.
Note: If you don’t have a TIN (taxpayer identification number) by the due date of your return
(including extensions), you may not claim the child tax credit (CTC) or the additional child tax credit (ACTC) on either your
original or an amended return, even if you later get a TIN. Also, you
may not claim the credit on either your original or an amended return
for a child who doesn't have an SSN by the due date of your return
(including extensions), even if that child later gets an SSN.
(Note: I expanded the acronyms in the IRS statement)
However, OP asked in a comment whether an extension would work if he could get his child's SSN by then. I turned up this FAQ from the IRS.
- File now, do not claim your child and later amend your return.
- File for an extension. You will still need to pay now any taxes that are due.
Question My daughter was born at the end of the year. We're still waiting for a social security number. May I file my return now and
provide her social security number later?
Answer If you file your return claiming your daughter as a dependent and don't provide her social security number (SSN) on your
return, the IRS will not allow you to claim her as a dependent.
You have two options:
You may file your income tax return without claiming your daughter as
a dependent. After you receive her SSN, you may then amend your return
on Form 1040X, Amended U.S. Individual Income Tax Return. Generally,
you have three years after the date you filed your original return or
two years after the date you paid the tax, whichever is later, to
amend your return. The other option is to file a Form 4868,
Application for Automatic Extension of Time to File U.S. Individual
Income Tax Return. This option would give you an additional six months
to file your return; by then you should have your daughter's SSN.
However, any tax owed will be due at the filing due date without the
In addition to the dependency allowance for your newborn, you may be
eligible to claim the earned income credit (EIC) and/or the child tax
credit/additional child tax credit (CTC/ACTC). Please note that you
can't count your child as a qualifying child in figuring the EIC or
claim the CTC/ACTC on either your original or an amended return if
your child doesn't have an SSN by the due date of your return
(including extensions), even if your child later gets an SSN. For more
information about taxpayer identification number requirements, see the
Instructions for Form 1040.