Thank you for your enquiry.
Your girl is here on a visitors visa. She is not able to switch visas in the country for work or study on a standard visitors visa. She will have to leave the UK and make her application from Brazil. If she succeeds in getting a student visa, she will be able to work 20hrs during term time as an au pair.
There are very limited options for non-eu nationals to work in the UK as an Au Pair. Your friend will need to apply for a Tier 2 work visa and be sponsored by an employer who has a sponsorship licence. Brazil is not one of the participation countries under the youth mobility scheme and therefore a Tier 5 (YMS) visa is not an option.