I would also stop providing extra clothes and tell the centre not to use your spares. It sounds like you are being lovely, but I am sure the centre will not leave her all day in wet clothes and it is up to them to find spares or contact the mother to bring more in or pick up the child. If the child is having that many accidents then perhaps the centre needs to be more vigilant in putting her on the toilet or changing back to pull ups if she is still miles away from fully trained.

It really isn't your responsibility and I can see that your intentions are good but it is only enabling the mother to be slack. She needs to sort it out herself.