Child tax credit: should I file as a non-filer?

Non-filers can enter their info here at IRS.gov, to start getting the monthly payments.

But I question whether or not you are truly a non-filer, if you were a dependent on someone's return. On another page, it says that the non-filer form is not for "someone who can be claimed as a dependent" on another person's tax form. In this article, you can explore whether or not you can be claimed as a dependent. Note that even if your benefactor does not plan to claim you as a dependent, that's a separate question from whether he can claim you.

It's too bad this is complicated. If you have time, you could make an appointment to visit an IRS Taxpayer Assistance Center.

Regarding the worry about being overpaid by the IRS, there is some protection from having to repay in the event of overpayments. Please see this FAQ, question 6:

Q H6. How do I know if I qualify for the full repayment protection for filers based on income during 2021?

First of all, if someone can claim you as a dependent (i.e. meets the conditions in the law to claim you as a dependent), then you cannot claim dependents and you cannot get the Child Tax Credit or stimulus payments. This is true even if nobody actually claims you on their tax return.

You said that he isn't going to claim you as a dependent for 2021, but that doesn't tell us whether he can claim you. If he can claim you but just chooses not to, you still cannot claim dependents and you cannot qualify for the Child Tax Credit or Recovery Rebate Credit. On the other hand, if he can't claim you (i.e. he does not meet the requirements to claim you as a dependent), and you can verify that you meet the requirements to claim your daughter as your dependent, including providing more than half of her support, then you should qualify for the Child Tax Credit, and you can file for the advance credit in 2021.