If you are planning to claim fewer dependents on your 2021 tax return than you claimed on your 2020 tax return, you should probably try to at least partially opt out. (I don't know if the IRS portal can allow you to unenroll one child but keep the other or if you have to completely unenroll.)
You would see a significantly reduced refund or even owe taxes if you received advance payments for two children but only claim one.
https://www.irs.gov/credits-deductions/advance-child-tax-credit-payments-in-2021
