Which deduction amount can I claim for the senior - filing jointly - only one child dependent?

For Single over 65, your standard deduction would be $14,050.

Most taxpayers have a choice of either taking a standard deduction or itemizing their deductions. If you have a choice, you can use the method that gives you the lower tax.

The standard deduction is a dollar amount that reduces your taxable income. It is a benefit that eliminates the need for many taxpayers to itemize actual deductions, such as medical expenses, charitable contributions, and taxes, on Schedule A (Form 1040). The standard deduction is higher for taxpayers who are 65 or older, or are blind.

2020 Standard Deduction Amounts

You do not have to "claim" a deduction amount---the software will take care of that based on the date of birth you enter in My Info.   It goes by your age at the end of 2020.  

Single $12,400   (+ $1650 65 or older)

Married Filing Separate  $12,400   (+ $1300 if 65 or older)

Married Filing Jointly $24,800   (+ $1300 for each spouse 65 or older)

Head of Household $18,650  (+ $1650 for 65 or older)

A dependent child does not change your standard deduction.   But depending on your sources of income  and the age of the child you claim, you might be getting certain child-related credits.