I agree, no diaper bags. It just isn't sanitary to bring them back and forth. Plus I don't want to have to search each bag for dangerous items (I worked at a center once and the Pre-K students had backpacks to take to/from school. One child had weed in his backpack
I don't want that happening on my watch)
You can either:
1) Not accept into care without enough diapers.
2) Increase tuition and provide diapers yourself. Honestly, at this point, this is probably what I would do. Send out a letter: "Dear Parent as of date your new tuition rate will be $ - with this increase I will now be purchasing and providing diapers for DCK."