Thank you JDBrits for posting about this, and for your willingness to help other au pairs.
Au pairs should also note that not all genuine families are willing to pay for visa costs, and some genuine families seek an au pair who doesn't require a visa. If a visa is required, however, this will need to be obtained by yourself from the host country's Embassy or Consulate in your own country. Always google to find the the relevant Embassy contact details, and do not contact email addresses supplied by the host family.
Also, many scammers do not include fake agencies in their scams, and many scammers do not request money for a visa. Au pair scammers invent many types of request for money, and may pretend that you need to pay for a work permit, residence card, health insurance, National Insurance number, tax clearance certificate, courier charges, deposit for an apartment - or anything else that they can think of.
Some scammers pretend that an au pair must make a Confirmation/Proof of Sufficient Funds by transfering money between a friend or a relative and themselves, using Western Union or MoneyGram. The scammer will then steal the money transfer, using the payment details which the au pair has supplied as confirmation.
To best protect yourself against scammers, please research the genuine regulations for becoming an au pair in your chosen country, and also what you can expect as an au pair (amount of pocket money, hours and responsibilities, etc) - and compare this to any offers that you receive from host families.
And never send money, for whatever reason, on the instruction of a host family or anyone that they have put you in contact with.