Do I have to insure and bill a babysitter even for a one-off assignment?

Yes, you also have to register your babysitter for a one-off assignment. However, there are two exceptions here: 

1. Your babysitter works on a self-employed basis and bills you.
2. It is a sack money job, i.e. the total salary in the calendar year is less than CHF 750 and the babysitter is no more than 25 years old in that calendar year.
   In these two cases, you do not have to register your babysitter for a one-off assignment.