TurboTax has the logic to process the payments. Something seems to have gone wrong with the entry for your child.
Please delete the entry for the child in the TurboTax interview. Then make sure that the form is not still in TurboTax.
You need to do this on a computer or in a browser. It will not work in the app.
On the left select Tax Tools
Tools
Delete a form
Find the form for the child and delete it
Enter all of the child's information again and see if the stimulus amount is corrected.
It is possible that there is an error in one of the other dependent's information.
Go through the same process with your other dependents until you get the correct stimulus payment amount.
If you are still unable to resolve this you can contact TurboTax for more help.
How to Contact TurboTax
