Be aware that you have no obligation to do so. If the au pair has come to you through a placement agency, it is possible that he/she has already subscribed an insurance policy, however, it is not automatic: it is wiser to check.
Cap Au Pair is the perfect policy for young au pairs.
You can subscribe online on her/his behalf. In this instance, you are “the subscriber” and she/he is “the insured”. You only need to provide the following three information about her/him: first name, last name and date of birth.
Payment is done by credit card, you will instantly receive the policy by email. You can forward the policy to the insured before her/his departure date so she/he has all the information when she/he arrives at your place.
This answer only relates to: Cap Au Pair.