Sounds like there are many issues posted here and unfortunately many answers will have to wait until the IRS sends the letters of explanation out in a couple of weeks. I believe in their haste to get the money out some mistakes could have been made ... many may have gotten money based on the 2018 return instead of the newly filed 2019 return (that may not have been processed yet especially if you owe the IRS) which would explain the missing money for the children born in 2019.
Unfortunately I doubt these issues will be corrected now ... most will have to be fixed when filing the 2020 return next year on the reconciliation form/worksheet.
