AuPairWorld checked directly with the German Federal Office for Migration in Nuremberg. Here is the answer from their press representative:
Most of the EU member states (including Spain) are part of the Schengen area in which border controls have been eliminated. For visitors from non-EU states, the same freedom to travel applies as for EU citizens, provided of course that the non-EU citizen is legally present in one of the EU states. A residence permit is required for a young person doing an au pair stay. As soon as this residence permit has been granted, the au pair can spend up to three months in other countries of the Schengen area..."
For details in specific cases, please consult the responsible immigration authorities.