Sounds like a lot of extra paper work on your end. If you charge a set daily rate you know what you are owed, the families know what they owe. Simple. Especially if you charge for care before the weeks of care. If you charge by hour then you have to keep daily logs of who stayed how long and will need to let every family know what they owe each pay period based on whether they stayed longer some days. Which would mean they have to pay after the weeks of care.

I think it's easier to just charge a daily rate and assume each child will be there your full day, if they leave earlier it's a bonus for you. You can shorten your work day if you know they don't actually NEED you open as long each day.