Take it as a lesson learned. I don't say this to be mean but she didn't MAKE you charge her less, you CHOSE to do so for whatever reason. Make sure you don't do this again and you won't have to worry about getting in trouble if your audited or a parent rats you out.

If you feel comfortable, ask her if she is planning on leaving or ask why she wants a receipt now. I would also charge her the full amount starting now if she is going to want receipts every year(which is her right).

(side note....why would a parent not want a receipt? Is there some reason why they wouldn't want to claim their daycare?)