Do you have a contract? If not, you should. Parents need to commit to and pay for any days their child should attend. So, you make them commit to however many days they need and WHAT days of the week those days are and they MUST be consistent (so, always Mon, Wed for example). If they don't attend they pay anyway.
I would simply tell them that since they NOW have changed from 3 days to two days that you need to re-evaluate the deal. Give them a CONTRACT, make them sign it to have care and then stick to the contract.
If you need a contract I am happy to email you mine. Simply PM me with your email address.