If you keep your 2 children with your ex wife in your information (so, included for 2020) but check the box that asks "can someone else claim this person as a dependent" so that the TurboTax system reads correctly that you are taking one this year and not the other, THEN add your new baby with your current wife, you will wind up with the 2 children that you are able to take on your 2020 return as dependents. Then, towards the end of your return, you will need to not only report the stimulus payments you did receive, but also fill out the claim for the one you did not (for the newborn in 2020). Per the regulations, if you had a baby in 2020, you are eligible for an additional $1,100 for the tax year 2020.
Here is a link that better explains this, from NASDAQ:
Parents With Babies Born in 2020 May Qualify For A Stimulus Check
