But this isn't normal kindy, this is privately-run pre-kindy for 3 year olds.... So I assumed they could make their own rules?? They are teachers, not child care workers, so they don't change nappies... I really wish this wasn't the case

I also sent my DD to a private 3 year old kinder, not a child care centre. No, they won't change nappies, but you won't send her n nappies. But they assist the child with changing in the case of accidents. If you have been to the doctor and she has had medical issues with withholding etc anyway, they certainly cannot discriminate. I would suggest you sitting down with the director and discussing the issue.

Have you tried removing the nappies altogether? As in, they aren't in the house so she has no other option but to go in the toilet?