I think the bottom line is this:
1. The IRS is so backlogged, you are lucky to get any advance payment at all.
2. The IRS has promised to create a web site where you can update your children, and enroll or un-enroll from the advance payments, but the site only partly works. If you find your children are not listed correctly, you might be able to update it in a couple months even if you can't update it now.
https://www.irs.gov/credits-deductions/child-tax-credit-update-portal
3. The advance payments are part of a credit that will be on your 2021 tax return. If you don't get the advance payments, or don't get the right amount, it will all be reconciled on your tax return. You aren't being cheated out of any money if the advance payment amount is not correct, it just means it will be added to your tax refund instead.
The advance payments don't change how much you get, it just changes when you get it.
