IRS doesn't allow you to add dependents' W-2 forms to your tax return. Since it's "earned income," the dependent would have to file his own return. If he's not required to file, he may want to file anyway in order to recover any tax withheld.
Here's a link to IRS Publication 17, where you'll see the applicable filing requirements under the Dependents heading on page 6: https://www.irs.gov/pub/irs-pdf/p17.pdf.
