This may help:
https://www.cdc.gov/coronavirus/2019-ncov/travelers/travel-during-covid19.html
https://www.cdc.gov/coronavirus/2019-ncov/vaccines/fully-vaccinated-guidance.html
To sum up the guidance--
Fully vaccinated people can:
-Resume domestic travel and refrain from testing before or after travel and from self-quarantine after travel.
-However, fully vaccinated people who have come into close contact with someone with suspected or confirmed COVID-19 are to be tested 3-5 days after exposure, and to wear a mask in public indoor settings for 14 days or until they receive a negative test result.
I would recommend you review these recommendations with your au pair prior to departure and discuss what the plan will be for her when she returns.
I am a medical doctor and have had several vaccinated patients return from Florida trips with symptomatic COVID in the past two weeks so this is not a risk free activity.