Good news is you can amend to add the child once they have SS# or file the W-7 ITIN application with the amended return.
Bad news, since you already filed the original return you will forfeit the CTC & EIC if eligible. Sadly the TT program will give them to you but the IRS will deny you those credits ... expect this to happen and for the processing of the amended return to take 6 months or more once the ITIN has been issued.
How do I file with a W-7?
TurboTax doesn't include Form W-7, so you'll have to download it directly from the IRS, fill it in, and print it out (see these IRS instructions for help with that).
Then, complete your return as you normally would. Leave the applicant's Social Security Number field blank, and disregard any warnings about the empty field or missing health care info for the applicant.
When your return is finished, go to the File section and follow the steps to print a paper copy of your return, as you won't be able to e-file with a missing SSN.
Bring your printed, signed return (along with any attachments per the filing instructions, such as W-2s or 1099s), the completed W-7, proof of identity, and foreign status documents to your acceptance agent or an IRS Taxpayer Assistance Center.
Alternatively, you can mail everything to:
Internal Revenue Service
Austin Service Center
ITIN Operation
P.O. Box 149342
Austin, TX 78714-9342
See also General ITIN Information at IRS.gov.
The Protecting Americans from Tax Hikes or PATH Act made several changes to the tax law, including preventing retroactive claims of the EITC and CTC by amending a return or filing an original return for any earlier year in which the individual or anyone listed on the return did not have a SSN valid for employment. You can't claim EITC or CTC unless the Social Security number for you, your spouse (if married filing a joint return) or a qualifying child is issued before the due date of the return including any valid extensions.
http://www.thetaxadviser.com/newsletters/2016/jan/congress-makes-changes-to-popular-tax-credits.html
