Here is the Senerio:

Taxpayer has 2 children from 2 different mothers. in 2020 Taxpayer filed head of household with Child (A) and received $1400 for Child (A) and $1400 for himself.

For 2021 Child (A) will be claimed my Mother (1) and not on Taxpayer's return.

Taxpayer will Claim Child (B) born in 2021 on his return and qualifies for $1400 stimulus for child born in 2021. 

How does the taxpayer claim the Stimulus for this child (B) when recovery rebate worksheet only recognizes for Child (A) dependent, that was removed for 2021 year? Child (B) replaces Child (A) but $1400 needs to be claimed for CHild (B)