If your taxable income has already been reduced to zero, then adding a dependent will not have any impact. You can check this by previewing your form 1040 and looking at taxable income on line 43 (instructions below). 

The dependency deduction reduces your taxable income by $4050, but as a married couple, you already got at least the Standard Deduction of $12,700 and two personal deductions for $8,100. 

There are some deductions that are based on having a dependent and how you answer the questions, within that section. From within your return, search each of the following topics (top right search bar): Child and dependent care, Earned Income Credit, Child Tax Credit

To preview your form 1040: https://ttlc.intuit.com/replies/3302322