Au-pairs from Bulgaria and Romania have to apply for an accession worker
card (Form BR3 can be downloaded on this page
http://www.ukba.homeoffice.gov.uk/workingintheuk/eea/bulgariaromania/applying/)
The Home Office website says it has to be done before arrival in the UK but Immigration at Heathrow say that in practice it's almost always done after arrival (within 3 months).

The Au-pair should apply for and pay for this herself.