Don't do it.

I know what you mean. Since your W2 did not list the 3600 FSA, you could claim 600 child care credit (3000 * .2). If you claimed the credit, most likely IRS would not find out. But how about your concience? when you fill the child care credit form, how do you answer the question "do you have a FSA?" 600 is not your money, and so do not "steal" it.