- I'll be needing care from around 6:30 to 6:30 2 days a week - am I likely to run into difficulties finding people willing to do these hours?
I think you may struggle to get people to start at 6.30am. If it were 7am it's a nicer time, though not much nicer. With early start times it is vital to check how far someone is travelling to your home, they will need to be quite local.
- is it reasonable to 'first round' interview while babies are in bed and then do a follow up interview with kids in tow? I'd like to see how they interact with my kids
Yes, not unusual for an initial interview to be in the evening, as many nannies work till quite late and would be coming to an interview after work. Then if that goes fine, you can then arrange a daytime meetup on a day that suits you both.
- where can I get contracts from? Can payroll companies help?
Yes, nanny payroll companies can help. This thread has some links and info about creating a contract/written statement.
- is it OK to ask what nanny is currently getting paid?
Why do you need to know that?
Advertise the job with the salary specified. Do not say it is negotiable as it probably is not very negotiable - you will have a set plan for how much you will pay. State in the advert the minimum training requirements, minimum experience you expect, along with the Gross on offer and then see who applies.
- what duties in addition to looking after the children are usually asked?
It varies. Children's laundry and bedding, stacking/unloading the dishwasher, cooking meals for the children, general tidy on occasion. Some nannies will do ironing, others will not. Specify expected duties in the advert. Keep in mind how much time the nanny will have, especially as children get older. You say "babies" so does that mean you have twins, triplets, quads?