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.
