blondeshavemorehavefun, while it is true that nannies are the only workers who are able to bring their children to work, their work is by no means comparable to a worker in an office environment. Nannies work longer hours, and have no lunch break, they cant just go our for lunch like the rest of us, so their job is quite different.
I dont think all nannies expect to bring their children with them, most realise they will be lucky to find a nice family who will realise that they will do a good job, despite bringing their own child.
Where I think a nanny with her own child worked for us:
She understood my fears and worries, as a mum
She understood when I was perhaps a little too concerned about the small stuff, but humoured me anyway
She was probably a bit more flexible I think, she just fitted right in
Not for one moment did I think her attention was divided. What happens when a family has more than one child, does attention not get divided then?
Your personal choice, do whatever you feel comfortable with.
Good luck.