Oh no- Not fun!! I don't do that, too much stress! I have one p/t kids and they need to commit to 3 day/ week and their days must be handed to me the Friday before the next week. If they don't let me know at least 30 mins before that they're not coming ( I think this is very reasonable) then they will still be charged for the full day and are unable to use it as a "sick" day.
I would be setting down some rules for them- hopefully they are more respectful towards you and your services in the future!