For myself I am open my set 9.5 hours and parents are free to use all of that time if they wish regardless if they are at work or not. I do have one family who I take their daughter casually providing I have room. Our agreement is pickup before 330 even though I am open until 5. I have never had an issue.

You need any sort of agreement in writing. What if you changed you contract to read that your daily fee is for 9 hours a day and that any child you is there for longer pays an additional fee?