You have lasted longer then I would have. After the first accident I would have told the parents that she has to be in pull up or no care for the day. I have a 20 day (daycare days) accident free rule. I also have a rule that if the parent sends the kid in underwear with out my write consent (you don't always get to check at drop off) then they are responsible for 100% of the reimbursement cost of every item/rug/furniture etc the pee or poo touches.

You have to protect your home and every ones health. Put your foot down on this one.

the school has the right to refuse her if she is not trained.