You will need to consult an attorney.
Regarding the allocation of the Child Tax Credit, while it may be based on 2020 filing, it is claimed on the 2021 return as an advance for the 2021 Child Tax Credit and Additional Child Tax Credit.
The IRS states at What You Need to Know about CTC, ACTC and ODC who is a qualifying child for the Child Tax Credit and Additional Child Tax Credit.
For more information about the next stimulus, see: Stimulus payment resources for your 2020 taxes
For more information, see: 2020 Divorced or Separated Individuals, page 8 for Dependents.
Coronavirus, Recovery Rebate Credit and Economic Impact Payments: Resources and Guidance.
Child-Related Tax Benefits Comparison.
