A policy that works says three things: how much notice you need, what happens inside that window, and how to reach you when something goes wrong.
The mistake is writing it as a threat. "24 hours notice or you will be charged in full" is technically clear and practically unusable, because you will not enforce it on a client whose child was ill, and a rule you do not enforce teaches everyone that none of your rules are real.
Write the one you will actually apply. A shorter, gentler policy you enforce consistently protects your calendar far better than a severe one you waive every time.