Polly, I am the worst in job interviews, so I supposed it is do as I say, not as I do!
As an employer, I have always looked for someone who is engaging, smiles a lot, and who is going to 'fit in' with my family.
Anyone who discusses pay, holidays, naps etc early on in the interview creates the impression with me that they are more interested in money / time off, than my children - If it is not raised by the employer, I would wait towards the end before discussing this.
There are loads of threads on MN with potential questions - I always found these a bit daunting! If I am interviewing, I would be asking about experience / qualifications, but more importantly, views on discipline, and what activities you would organise for children(recent threads have been discussing the fact that some nannies rely heavily on paid activities - I like people who are going to bake, do craft etc with mine.
Nannies usually prepare dinner for the kids, so I would also ask questions about what type of things you like to cook - don't answer turkey twizzlers!
Finally, there is nothing wrong with saying to the employer that you are nervous! Some people will appreciate the honesty.
I also watch to assess the interaction with the children.
Good luck with the interview.