Do you understand how the RRC/EIP eligibility works? Because this: "if the child filed as independent for 2019"

Has no bearing on this: "and received stimulus payment?" or on the reality of the 2020 filing. What is supposed to happen is that tax year filings are accurate and factual. If that 2019 filing meets those conditions, there is nothing wrong with 2019.

For the EIP/RRC, the tax filings on file at the time the payments were made, were used to project what the 2020 filing would be. That's why the person got funds. Projection is not reality.

When you work on their 2020 filing, you abide to accurate and factual data. It doesn't matter if they got something they are not actually entitled to based on a faulty 2020 projection. On the 2020 filings, you do what applies. If that also includes showing the person is eligible for RRC, then you need to enter how much they already got. That's all that matters for the RRC, for 2020.

Prepare their 2020 tax forms as applicable to their status in 2020.