Ok ... first the IRS doesn't send out state refunds ... only the state does that.
Next ... if the 2020 return has not been fully processed yet the address has not been updated yet in the federal system so the 3rd stimulus was sent out based on the 2019 info they had so they did not send for the one child.
Now ... will the IRS send the missing stimulus money when they do process the return ? If last year taught us anything is that the answer is maybe but maybe not. If you don't get the missing stimulus money in advance then it will be a credit on the 2021 return you will file next year just like the first 2 payments were caught up on the 2020 return. So all you can do is wait patiently while the understaffed IRS tries to do more again with less.