Deductions & credits

First you must amend the 2020 return to get the child credits on the 2020 return ... and when you get to this screen READ the instructions and claim the WHOLE year ... screenshot says 2017 but the same goes for 2020.

Then for the advance CTC payments ... you are too late to get them now so all you can do is wait and get it all on the 2021 return you file next year.