Dd went on aupairworld.net and found a great family in Spain. So much so that when she came home, her sister went out in her place! They both had a great time, made loads of new friends and improved their language skills
Honestly? I wouldn't. I was an au-pair for a family friend and it was ok pretty lonely and I was at university so I had friends. We all had au-pair jobs (notoriously expensive city) and without exception all had a mixed time. And we were 20+. Is she doing it as a gap year before studying languages at university? There'll be a chance to spend a year abroad then with more support than doing it alone age 18.
Can anyone recommend an agency or other route to becoming an au pair in France or possibly Spain? This is for my 18 year old DD, who is about to start a gap year. She could consider short placements in either June or July, or something a bit longer in the autumn.