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.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
