There have been news articles recently where the police have warned parents not to leave their children alone in the car. People have been charged for doing so. I would be extremely hesitant to leave the daycare kids alone for even two minutes, and you would need permission from the parents for anyone other than yourself to watch the kids (if you take turns with others watching the kids in the cars). You need to make sure the daycare parents know exactly whatever plan you decide to follow and get their written permission to do so. But I wouldn't be asking the parents permission to leave the kids totally unattended in the car, as it is technically illegal.
Just google 'Is it illegal to leave children alone in the car in canada' and a bunch of news articles will come up about it.