The amount withheld from the distribution was only an estimate, just like withholding from a paycheck. The exact amount of tax is not known until you prepare your tax return, because it depends on how much other income you have. The distribution is added to your other income and taxed at whatever your marginal rate is, and the early withdrawal penalty is added, if appropriate. The total amount of added tax may be more or less than what was withheld. You get full credit for the tax that was withheld at the time of withdrawal. You aren't being taxed again, just once accurately.
The only way to get a refund of overwithholding is to file a tax return.
You can find the detailed filing requirements for a dependent in IRS Publication 501, at this link:
<a rel="nofollow" target="_blank" href="
http://www.irs.gov/pub/irs-pdf/p501.pdf">http://www.irs.gov/pub/irs-pdf/p501.pdf</a>