To claim your child as a dependent on your tax return, they need a valid Social Security Number (SSN), Adoption Taxpayer Identification Number (ATIN), or Individual Taxpayer Identification Number (ITIN). However, to claim certain credits, such as the Earned Income Credit, your child will need a valid SSN.
If your child isn't eligble for an SSN, you can apply for an ATIN using a Form W-7A or an ITIN using a Form W-7.
If you don't have any of these TINs for your child, you will need to file without claiming them as a dependent. Once the number has been assigned, you can amend the return to add your child.