My understanding of the CARES Act was that tax year 2020 is the governing tax year.

If there was an overpayment to the mother based on her 2018 or 2019 tax filing that was used to determine the initial refunds she get to keep the payment and no adjustment or payback by her is required.  For 2020 if the father now qualifies to claim the children, he is entitled to the credits.   Perhaps you really need to check Due Diligence requirements and verify that the mother qualified to claim HOH status for the prior years.