The first thing to get settled is whether you are going to be an "au pair" which has a rather strict meaning (limited hours per week, learning the local language, reduced responsibility for the children, normally a younger person etc.) or more a "house employee" which means more hours per week (full time), more responsibility for the children, significantly higher salary etc.
Often (unfortunately) employers will try to pass the job off as an au pair level one while expecting you to work 45+ hours per week with full responsibility for the kids, running the house etc. This is not fair and is not legal. Get the expectations clear right away and make sure you will not be taken advantage of. Slavery died out a while ago but certain families in CH missed the announcement
Then you need to make sure that the family will do things legally (a real contract with salary and duties, payment of the social insurance payments and insurances and declaring you to the local authorities so you have a permit etc). It is in your interest to work with someone who does things above board. There are enough of them so don't accept just any offer/conditions.
Good luck with the discussions.