You will find that there is a lot of information in this section of mumsnet about employing a nanny. It isn't that complex often, though some situations can arise that complicate things - which is where having the services of a payroll provider may be of help, though if you know people who are already knowledgeable about PAYE, statutory payments and funding, then you could certainly do the payroll yourself. A payroll service will typically cost less than £150 a year, if you are wanting them to produce Monthly payslips, remind you of when to make payments to HMRC, produce P60, P45 and provide general advice.
Whilst a nanny becoming pregnant or going on paternity leave could happen, how often does it really happen? Nannies may go on long term sick but they may not get long term sick pay... many nannies only get Statutory Sick Pay. It's needing to find a temporary replacement that is the costly part in my view.
However ask nannies on here and you will find that many of us will say that we have rarely had a sick day, yet along been on long term sick, or even become pregnant (or taken paternity leave). Whilst these things could happen, you deal with it at the time it occurs, I would not worry about it now as it may never happen.
I would suggest you look carefully at the costs involved - use MrAnchovy's PAYE Calculator to get a feel for how much you will pay in Employers NI based on a given Gross salary. Calculate some likely day-to-day costs - activity budget, likely travel on duty, providing lunch and drinks.
Search this part of mumsnet using Advanced Search using keywords like Contract to find possibly useful message threads. Here is quite a recent message thread which includes a link to a contract that you could use as a starting point.
In my view, with 2 children a nanny is likely to cost a bit more than nursery... though brings advantages, like having the children's washing done and not having to drag your children out of bed in the morning.
If you are considering taking on someone who has only done nursery work before then be aware that it can be very lonely being a nanny and very full-on. A nanny does not get breaks like a nursery worker does, or backup support from others around them. A nanny is the only person there... they have to cope with whatever happens, with the children and the home. Some nursery nurses adapt well to nannying, others do not. If they have done lots of babysitting including some daytime care such as at weekends, that may help convince you that they can cope with being a nanny.