Since Brexit, UK citizens no longer have an automatic right to work in France. However France does still issue long-stay visas to foreign nationals wishing to work as au pairs, who're aged between 18 and 30 and where this formal contract is in place between the au pair and the host family:
https://tinyurl.com/52w5hdw3
So, whichever route your daughter takes to finding a suitable placement, it's absolutely essential that she ensures that such a contract exists before applying for a visa. Information about applying for a visa can be found here:
https://france-visas.gouv.fr/en_US/web/gb
Your daughter is more fortunate than a French citizen wishing to work as an au pair in the UK, as that's now effectively impossible. (The UK abolished 'au pair' visas in 2008. An EU national wishing to work here as an au pair would need to be able to prove that they were a 'skilled worker', with a salary far in excess of what au pairs usually get paid, in order to obtain a work visa).