I have all stat days listed in my contract and parents know they are to pay for them at the discount of $10 less/day. I have 11 in my contract and those are the only ones I get paid for.

If you don't already have something like this in your contract, I would suggest adding it in for future reference. You own your own business so you can essentially take off whatever stat days you want but to be fair to your clients, they should know these days in advance.