dependent care fsa

FSA. 

The credit is non-refundable, meaning that no matter how large the credit is on paper, it won't be paid if she doesn't owe tax to begin with.  Assuming she files as single or head of household, and with 2 child dependents, the standard deduction and the (up to $2000) child tax credit will completely offset any tax she owes, so she would not actually get any money for the childcare credit.  The FSA will save her money because it reduces her taxable income before the tax is even calculated.  If she is married and her spouse works, then the FSA is still the answer, because at a higher income level the FSA saves more money than the credit.  

Unfortunately, one would have to make a practice tax return to be sure.

But, in general, I think it is true that those with small incomes benefit more from the credit while people with high incomes (i.e., higher tax bracket) may do better with income deferral (which is what the FSA does - defers income to pay for expenses).

But, as I said, this is just an estimate or generalization - do a practice return, change some numbers, and see how it turns out.