I charge per month and it includes all sick days or days that they stay home for whatever reason. Also, payment is due at the beginning of the month, this way they won't be reducing the payment for the days they missed. I find this was works best. I haven't had a problem with payments yet. I make sure to go over the contract with parents in detail first before they sign to avoid confusion or arguments later on. What you could do is give a reminder note a couple days before payment is due and just say this is a remider that payment of (whatever amount they owe) is due on (date).
Good luck. And let us know how it goes.