They enhanced Child Tax Credit is not retroactive for 2020. Rather, it takes effect for tax year 2021 (returns filed in 2022), so I'm not sure where that $3100 is coming from. Please check everything carefully.
But only the person with whom the children lived for more than 50% of the year can claim them, unless the Custodial parent releases this to the Non-Custodial parent via Form 8332.
About Form 8332, Release/Revocation of Release of Claim to Exemption for Child by Custodial Parent |...
