This is from the Virginia's DMV  

"If you are visiting in the U.S. from a foreign country, you may drive using your driver’s license issued by your home country. This license should be accompanied by an international driver’s license which provides a translation of your license. International driver’s licenses marketed by private sector businesses do not allow you to legally operate a motor vehicle." The website where this can be found is: http://www.dmv.state.va.us/webdoc/citizen/drivers/vadm/vadm1-4.asp

This law is for visitors, however, au pairs are more than visitors.  The au pairs can drive when they first get here with their home license and international license for at least 6 months. 

(Just a side note, after a very long conversation with the DMV I was told that au pairs may drive with their International Driver's license for the entire year, as long as it has not expired... Please note that your insurance may still require the au pair to get a Virginia License to be covered on your plan)

As of January 1, 2004 it is now more difficult for au pairs to obtain their licenses due to Proof of Residency laws.  It now looks like the only form of proof of residency that an au pair can easily obtain is a bank statement.

Testing

Au Pair from Germany, Canada, and France are exempt from having to take the written and driving tests. (However, getting their license may NOW be delayed because they contact the au pair's country first to verify the license is legal, and then send you a letter, so expect 2 trips to the DMV).

All other au pairs must taken the written test before getting their Virginia license.  The test is available in English and Spanish.  You may want to pick up a study manual from the DMV for the au pair prior to her arrival.  There is also a place on the website to practice the test.  After passing the written test, a driving test will be required.