I'm a nanny with a child and I find some of the points made here a little frustrating. It isn't difficult for someone with childcare experience to look after 3 children and you will probably find, after the initial settling in period, that the children all get along very well and it is enriching for all of them.
The children I look after are older than my child and we only go to activities which are appropriate for the children who I am paid to look after. If my child is required to pay to be there then I would of course pay that. I would always prioritise them in this way and do activities more suitable for my child in the days when I'm not at work. The only thing I am really unable to do with them is go to the cinema or swimming without help, but we do go as we have another adult who can come with us.
Emergencies such as 'both children running out into a road', is really very unlikely to happen. In areas where this would be possible, the nanny would have thought ahead and put one or both younger children into a buggy or similar. Many teachers are able to teach their own child in a class of 30, without the education of 29 other children suffering; or run a cubs/brownie group containing their own child without showing favouritism.
My child hasn't been ill in the time I have worked at my job but I don't think it's fair to discriminate on this point. Should anyone who is a mother not get a job in case they have to take time off when their child is sick? If they were to fall ill, I would phone my employer and explain what was wrong with them and ask if they were happy for me to bring them to work. If not, obviously either myself or my partner would have to take a day off. If a child you look after is sick, this doesn't affect anything. I would still go to work. If my child picked up a bug from this it would be no different to them picking one up at nursery.
Also, I don't get paid less than I used to before I brought my child to work, although I suppose I haven't had any inflation-related pay rise so you could argue it is slightly less.
You should employ whoever you feel most comfortable with but I really don't think that is has to be seen as a negative thing to have another child around.