How do the children feel about having an au pair? / How do/did they feel about the previous au pair leaving?
You want to have time to mentally prepare yourself to be able to handle comparisons and parallels that the children might initially draw between you and the previous au pair. If their experiences with the previous au pair were negative, then you will need to work towards overcoming the negative image left behind. If on the other hand the children had a super relationship with the previous au pair, then you can expect to have them occasionally make you to feel like you could never be as good as the previous au pair... But hey, don't sweat it: all that fades away with time.