Per the IRS Instructions for Form W-2 (Instructions for Employee, Box 10):
Box 10. This amount includes the total dependent care benefits that your employer paid to you or incurred on your behalf (including amounts from a section 125 (cafeteria) plan). Any amount over $5,000 also is included in box 1. Complete Form 2441, Child and Dependent Care Expenses, to compute any taxable and nontaxable amounts.
To enter child and dependent care expenses, click