In order to report a child on a US tax return, the child must have US citizenship in the year of reporting and a Social security card by the due date of the tax return including extensions. For example, if a child was born in May 2023 then the child must have citizenship by Dec. 31, 2023 and the Social Security card must have a date no later than Oct. 15, 2024. If the card is dated after April 15, 2024 but no later than Oct 15, 2024, the 2023 tax return MUST be received by the IRS by the due date of the tax return including an extension by Oct 15, 2024.