I had had many aupairs for the past 15 years. I am form Denmark, so they were always danish, in most cases they have been an amazing help, but some times one did not work out, and I sent them home or rather helped them choose to go :) Treat them more like an exchange student and not like a maid and you will be able to greate a loving family like environment for your children. many of mine stil saty in touch, three have come back again, one stayed for almost 3 years, and I have also had guys do the job...being my first two kids are boys.
any question you may have I can answer.. I am sure I have had the experience.
One thing... I never went tru an agency, but have a close friend who " manages" aupairs in NY, so i know a little bit about that scenario.
send me some specific worries and hopes you have.