Update - we saw the nanny with child, without her child, last night. I liked her a lot, I think I would trust her with DS and I think she's probably a really good nanny.
BUT it was really disappointing. I said 'I've mentioned before that I had concerns about having a nanny who wanted to bring her own child, and I've turned down other candidates for that reason. Have you thought about what you think the main challenges of that situation would be and how you would address them?'
I thought that was the obvious question that she would be expecting to be asked, given she already knew I was a bit iffy about it. But she hadn't thought about it, she just thought 'oh well it would be fine, people manage with twins all the time'. She mentioned routines but didn't think it would matter at all if the babies didn't sleep or eat at the same time. I prompted her about logistics, equipment for her child. She said she brings high chair wherever she goes anyway, so that's fine, and obviously it would mean a double buggy which is a bit harder to manage than one (er, where is this double buggy coming from and it is being stored in my hallway along with single buggy all week?), and her DD 'sleeps anywhere' when away from home so that's not a problem.
I prompted her to think about what issues she thinks could arise now or in future from looking after her own child while at work looking after an employer's child - it's possibly not the same as being nanny for a family with two children. She couldn't see any potential issues. My DH asked her what difference was she finding between looking after an employer's child as she has done lots before and looking after her own child which she is doing now for the first time. None, apparently.
Of course I realise all these potential problems, logistical and otherwise, are mine as well if I were to choose to employ her, but I was just really disappointed that she hadn't thought about it, wasn't prepared for it, and even when prompted, couldn't see that any of the potential issues you have all highlighted above could arise.
My instincts about her as a person and a nanny were good, but my instincts about how she'd manage this situation were poor, because she just didn't seem to have even thought about it and wouldn't acknowledge that there might be difficulties for her and me.
Arrrrgggghh!
AND I've had another cv in from my ad in Gumtree, all looks great, until DH pointed out that this one hasn't got a proper driving licence, only an automatic one so she wouldn't be able to drive our car.
Good grief!