@JOHNNY3
It is important to understand that all of @Carl ‘s information will apply to 2022 and later, but does not apply to 2021 because both parents are custodial parents under IRS regulations for 2021, because both parents and the children lived in the same home for more than half the year.
All the referenced information that applies to 2021 and 2022 and after, can be found here.
https://www.irs.gov/pub/irs-pdf/p501.pdf
