When you come to federal review on your 2020 return you will have the opportunity to reconcile your stimulus payments. 

Yes, a child born in December 2020 can be included as part of your economic stimulus payment, assuming the child is your dependent. TurboTax will calculate the Recovery Rebate Credit for you.

To add your qualifying child to your tax return, go to My Info from the left menu and select Add a dependent. For more details, see How do I add or remove a dependent?

The stimulus payment section is at the end of Other Tax Situations.

To return to this area of your return, follow these steps:

• From inside your return, click Other Tax Situations (the third tab from the left toward the top of your screen)
• Scroll to the end of this page and in the lower right corner, click Let's keep going.
• Be sure that an amount is entered for each stimulus (or zero) so that the calculation is correct.

For more details, see IRS Form 1040 Instructions regarding Recovery Rebate Credit.