The dependent care credit does not care how the money is spent, the maximum eligible expenses are $6000 for two children, so you would report $6000 of eligible expenses even if you paid $600 for one child and $8000 for the other child.
However, the credit is not the full $6000 of eligible expenses, it is a percentage. 35% at low income gradually dropping to 20% at higher income. Meanwhile, your income tax is 10-15% at low income increasing to 25% at higher levels. And the FSA saves you 7.65% social security and medicare tax and it saves you on state income tax (3-10% depending on your state and income). Which is why the FSA will save you more money if your overall income is more than $20,000. The FSA saves you 15% + 7.65% +5%; or 25% +7.65% plus 5%, while the credit saves you 20%. If you have more than $5000 of eligible expenses, the tax form will automatically put the last $1000 toward the dependent care credit of 20%.
The only reason to prefer the credit over the FSA is if you are worried that your circumstances will change and you won't spend the full $5000, because unspent money in the FSA is forfeit, or if your income is less than $20,000, so that the credit (30-35%) is more than your tax savings (27.5%).
