Earned Income Credit & Child Care Credit

It is my understanding for the 2020 and 2021 tax years, you can use either your 2019 income or your 2020/2021 income to calculate both the EIC and the Child Tax Credit.  

1. Is it correct that this option is for for both 2020 and 2021?

2. If so, is it one or the other or could you use 2019 both for 2020 and 2021 if was most beneficial.

Thank you.