I demand pull ups until I hand them a written letter stating that their child can come in underwear. The letter also states that if their child has an accident well in underwear, they have to go back into pull-ups until presented with another letter saying we can try again. (I have had a few families try and say I said it was ok to stop bringing pull-ups when I didn't, so they now get it in writing).

I also have it that if they break my policy and send their child without a pull-up (unless letter has been given) then they have to pay 100% of the replacement cost anything the child's pee touches. This includes my couch, bookcases etc. Lets just say since I started this policy I haven't had anyone not send pull-ups until they got a letter saying its ok to do so. Not that I would actually charge them, but I have the option to do so if I feel its necessary.

Good for you for standing up for your policy and telling them to either fallow the rules or go else where.