Hi there,
I guess it can be a bit daunting to become an employer.. it's certainly given me a few headaches, but it has also been a good thing too. Having a nanny really suits us, but you do have some more responsibilities with it, especially if someone is giving up another job to come to you.
My advice is that you must draw up a clear, specific and formal contract of employment, and make sure both you and your nanny are fully aware of and understand the content before your nanny starts with you.
I'm not an expert on employment rights, so you'd need to check things out, but my basic understanding is, if your needs change - ie you no longer need a nanny because your children have gone to school, or you've stopped work, or you decide on an after school club etc. then the position becomes redundant. If your hours change, then I think that's fine too (you can offer your nanny reduced hours, and she can take them, or you can offer redundancy).. Like I said, I'm not an expert though so I would clarify. I've always had payroll done by an agency, which is really good, because most are able to advise too.
If your nanny decides not to return after maternity leave, then that's her prerogative (sp?) - she gives you notice that she's not returning as per the appropriate timeframes, in line with the maternity leave regs etc. If you decide that she could bring her baby back to work with her, its a subject where there's no hard and fast rules... If you decide it's not appropriate for her to bring her baby with her (pros and cons) then you can stipulate that, and she can decide whether to have her baby looked after and return or give notice.
There is quite a lot you could think about it.. but you could also 'over' think it.. If you've already decided that having a nanny would be a suitable, economic and practical option for you, other than agreeing the terms and formalising the contract in advance, everything else is subject to "see what happens" I guess...