For the parent, it is necessary to observe. Sure, you hired who you believe to be the better of all the other candidates, however, personal observation is invaluable. Does the nanny fluster when the baby cries? Can she multi-task? Is she organized? Is she self-assured? Ideally, it would be best, to observe a prospect BEFORE you hire, however sometimes that is not possible. Is she loving toward the children?
For the nanny, it is basically "on the job" training for 1 week or 1 day. Here's your opportunity to shine. Sure, you don't know where everything is, or the baby cries and the toddler is upset and you don't want the parent to think, you can't handle it. On the most part, most reasonable parents know you are training so as long as you do what you do best, provide exceptional care of their children, a parent won't hold it against you, if things are not perfect. We've learned in our profession to always, expect the unexpected when working with children! That is what makes being a nanny so much fun.
For the children it can be quite stressful. The older the child, the more stressful for them. Here you are, in "their home" trying to act like their mommy or daddy. We know that is not our job but from a child's point of view, they don't get it. The good news is, children are resilient and soon realize, you are not there to replace but to be there for them. Their own special friend. What a joy it is, after those first few days, when they call out your name, want to snuggle and ask for you, when you are gone.