I don’t have kids so I can’t do it, but I went to the id.me website. It is used not only by government but also private agencies. Looks legit but not sure of the point of opting out; sounds like if you do you’ll claim the credit when you file your taxes.
This is the section about verifying for government reason; scroll down about halfway where they show the process. https://www.id.me/business/government
https://www.irs.gov/credits-deductions/advance-child-tax-credit-payments-in-2021 for those who wonder what the advance payment is about.
“Looks legit but not sure of the point of opting out; sounds like if you do you’ll claim the credit when you file your taxes.”
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The point would be to continue getting the credit when filing taxes at the end of the year instead of getting it spread out over monthly installments in advance.