Your question is unclear. There's a difference between your return being filed, or rejected, or being held awaiting verification.
It sounds like you filed your return, someone else also claimed your children as dependents, the IRS requested proof, and after reviewing the proof, they took the other person's side and removed the dependents from your tax return.
In that case, the IRS determination is likely final as to 2020. The fact that you were able to prove your case for 2021 does not mean you can retroactively re-open your case for 2020. If there was a determination against you, there is a specific appeal process with time limits. You can appeal to the examiner's supervisor, and then you can appeal to the Tax Court and have a hearing. If you missed the time limits to appeal, your 2020 case is closed. If you are within the appeal time limit for your 2020 return, you would have to file your appeal through the correct appeals process, which is not the same as just filing a new or amended tax return.
On the other hand, if your 2020 tax return was rejected electronically and you never re-filed it, then you never actually filed a 2020 tax return and there has been no official determination of your situation. You can–and should–file your 2020 return by mail whenever it is ready.
In the third possibility, you e-filed a return that was rejected, so you removed the dependents and filed without them. In that case, there has been no determination about their status because you never actually claimed them so the situation was never reviewed. In this case, you would file an amended 2020 tax return to add your dependents. If there is a problem, the IRS will begin an investigation.
