Seems very odd that it was accepted with incorrect SS of the child. I suspect it was because of the advance CTC, and not the SS number. Probable a new hire at IRS. I would call the number on the letter from IRS and speak with the agent. I would also ask client to provide the child's SS card, and last years taxes if child was claimed last year. As for the 1040x if it turns out the SS was incorrect you do need to file 1040x, You are only changing information. If you aren’t changing
any dollar amounts you originally reported, but are changing
information and the rules in Resident and nonresident aliens,
earlier, don’t apply, do the following in respect of your Form
1040-X.
• Enter the calendar year or fiscal year you are amending.
• Complete name, current address, and SSN (or ITIN, if
applicable).
• Complete Part I, Dependents, on page 2, if you are changing
your dependent.
• Check a box in Part II, Presidential Election Campaign Fund, if
applicable.
• Complete Part III, Explanation of Changes