If the child will be reported on the other parent's return for 2021 then the parent who claimed on the 2020 return and is getting the advance payments should OPT OUT ASAP since they will need to repay the advance payments on the 2021 return if they don't get the credit.
The parent who claims the dependent on the 2021 return will get the credit on the 2021 return OR eventually they may be able to use the IRS portal to indicate the dependent will be claimed by them to get some of the credit in advance.
READ all the FAQs here : https://www.irs.gov/credits-deductions/advance-child-tax-credit-payments-in-2021
