Occasionally, I see someone on these sites that cite their experience as an au pair. If they are still an au pair, then they are working illegally because the terms of their visa only allow them to work for their host family.
If you're ok with hiring nannies under the table, then I'm sure this is fine for you too.
However, this is a lesser known issue that many people don't know about. There is NO legal way for an AP to convert their visa to stay and work. It's about equivalent to hiring someone on a student visa, who isn't allowed to legally work for anyone other than work-study/fellowships at their school.
-Signed someone who stays away from these sitters