I think it really depends on parents' situation. Couple of years ago, when weather was bad both my and my husband's employer gave us "admin leave". Now, it is either dip into your PTO or telework. Our employers don't really care if you have child care problems -- if you do then take PTO. Annoying? Yes. It is obviously cost saving measure, but not the reason for either go us to quit. As pp mentioned when you are professional -- deadlines don't move so actually we prefer to work.

Our previous nanny lived 1 hour away and had a pretty unreliable car, and with forecast of like 2in of snow declared that she can't get out of her house. We gave her "admin" leave and one of the parents would then take PTO. it was annoying but we did not have a choice.

Our new nanny drives my van that is in very good condition and lives about 5 min away. She has good experience in driving in snow (Russia). So far, I only gave her a delay couple of times enough to defrost and clear the car before coming here, and once i went to pick her up in our AWD SUV. Obviously, if there is like 1ft of not removed snow, we would tell her not to come and then give her "admin" day, but if I feel that it is safe to drive and can pick her up, there is no reason she shouldn't come.

So, I think what to put in the contract depends on the circumstances of the nanny, how far away she lives, how reliable is her car, how comfortable she is with snow, etc. we didn't have anything regarding snow days in the contract.