A couple other thoughts. These girls aren't 3 year olds, in which case as a parent I would hold you responsible for making sure they were warm enough.
These girls are 9, and fully capable of making their own decisions about jackets, IMHO. This is a power struggle you're in with them, and they're winning.
I wouldn't babysit for kids whose parents would call me useless over their kids not complying with simple (but kind of unnecessary) requests.
Best wishes.
Why do all three of these children not want to wear their jackets or zip them?
I've seen this behavior in boys, in about 3rd or 4th grade, who try to be macho and pretend they aren't cold, and the teachers deal with that by saying if you don't bring and wear a jacket to recess, you can do recess in the library instead. And that's that. Next day, they all have jackets.
I would just calmly ask, why do you not want to wear your jacket? I raised 3 boys and had countless kids come over to the house, stay all day, go in and out, and I've never had an issue once with making a kid wear a jacket who didn't want to. You don't wear a jacket, your lips turn blue and you shiver. That's how that works.
They are supposed to wear AND fasten up the jackets (at the request of the parents) anytime they go outside.
So that is when they leave to go to school, at break and lunchtime at school, coming home from school and when they get picked up from mine to go back home.
Don't understand. When are they supposed to wear the jackets? When they leave? Go play? All the time?