Well, it's terrible to ask her to work on a Sunday.
Let her go out or rest from her week.
Not a nice way to welcome someone at all.
Then, just take her out for coffee without the kids and ask her how she feels, why she's being unhappy, what you can do to make it better etc ... Maybe it's just being away from her family (she can call them), maybe she would like to have friends nearby etc. Maybe the children are tough with her because they miss the last Au Pair and need to be nicer to her.
It can be a lot of things. It's worth finding out, you picked her for a reason and it's the beginning

good luck