A driving permit or an international driving licence is in Germany valid under the circumstances just for a certain period. Please get informed at your police station or at ADAC. You should be also advised there whether there is any necessity of an additional examination.
Make it clear with your insurer who would be responsible for damages caused by your au pair.
You can also give a chance to your au pair to take part in driving lessons in order to become acquainted with the transport rules in your country. When an au pair comes from a land where a man drives in the „false“ road direction, it is to recommend to make her/him take driving lessons before she/he drives your car.