Be careful when looking at websites offering visas (for anywhere in the world). The vast majority of them are commercial sites, that have no links to the relevant consular authorities. So you risk being ripped off by paying for more than is necessary and, if the agency that you're using doesn't do a good job, ending up with invalid documentation.
The link provided by Barry goes to a commercial website, which I wouldn't touch with an infinite number of the proverbial bargepoles. This is the official website of the Spanish Consulate in London:
http://www.exteriores.gob.es/Consulados/LONDRES/en/Consulado/Pages/Visas.aspx
Scroll down to, and click on, 'I want to go to Spain as an au pair. Do I need a visa'.
As you'll see from that link, the visa required is effectively the same as a student one, with the documents that are required being listed here:
http://www.exteriores.gob.es/Consulados/LONDRES/en/Consulado/Documents/SSU-SLU%20ES-EN.pdf
Once your daughter is sure that she has all of the relevant documentation, she needs to book an appointment at the Consulate. The relevant information is in my first link above.