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@Sandra1 wrote:

He clearly didn't take the CTC as its not marked or calculated on the return but he did get the $1400 recovery payment.

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I'm intrigued at how he received the Recovery Rebate but not the Child Tax Credit.

As was mentioned above, her E-FILED return will be rejected.  However, she could file by paper (if the IRS ever opens their mail).

Is Dad reasonable, wanting to correct things and willing to do what is best for the overall situation?  One possible option is for your client to give Form 8332 the Dad and have him to amend to claim both kids (including the child tax credits) and then share the extra refund with Mom (due to the Covid-related rules for 2020 and 2021, that would likely result in a better overall result than if each parent properly claimed one child) .  Your client would need to file by mail to claim Head of Household, EIC and Dependent Care Credit.