The explanations are contradictory. See this one:
"Taxpayers claiming the Child Tax Credit must now have an SSN or an ITIN by the duedate of the tax return (including extensions). Further, the child claimed for the credit must have an SSN, an ITIN, or an ATIN by the due date (including extensions). Taxpayers cannot make retroactive claims for the credit for a period that the taxpayer did not have a required identification number. This applies to any return filed after December 18, 2015."
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