How can I tell (without officially amending) how much my refund would have been reduced on federal and state taxes if I had only claimed one child for tax credit?

[xmasbaby0]

There is not an easy answer for that at this point.  You are too late to use the online software even to prepare a "dummy" 2019 return claiming only one child.  The 2019 software is closed permanently now.  When online re-opens it will only be for 2020 returns.

And messing around with your already filed 2019 return will make a mess of it and cause problems for you when you want to transfer it to a new 2020 return, so that would be a bad idea.  Amending would give you the information you want---but only if you are adept enough to get out of amending without doing permanent damage to your return.

If you used desktop software you could save the return and then "play" with it--but you posted from online--so it does not appear that you have that option.

[rjs]

Even before TurboTax Online closes for the year it doesn't have a good way to do what-ifs like that. In TurboTax Online there is only one copy of your tax return, and any change you make is saved immediately. There is no back-out or undo, and no way to exit without saving the changes.

If your tax return is not too complicated, you could enter the information in TaxCaster with only one child and see how it comes out. TaxCaster is still set up for 2019. But TaxCaster is only for federal, not state.

If you were using the CD/Download TurboTax software, besides being able to play with a copy of your tax return without affecting your actual tax return, the software has a What-If Worksheet that lets you easily try out different scenarios using the completed return as a starting point, but without affecting the actual tax return, and it lets you compare the scenarios side-by-side.