it doesnt matter what status you filing with MFJ/head or single.
but to claim kids benefits it depends when you got SSN or ITIN for kids. ITIN received later/current cant be used for prior year CTC.
SSN if received in earlier year you can still use to get child tax benefits.
2020 - not sure.. check IRS terms.
https://www.irs.gov/filing/file-an-amended-return#:~:text=When%20to%20file%20an%20amended,from%20the....
2021. might be possible
