Yes, I would put out an email notice immediately stating that effective immediately, every 5 minutes (minute, or however you want to work it) a parent is late, you will charge X amount. I would further say that everyone must acknowledge the email by replying before the child can return to care.

That is completely disrespectful. I also can't believe she TOLD you instead of asking you. And I also can't believe the gall of them completely disregarding that you refused the extended hours, and just didn't bother showing up.

Personally, I would terminate them. I think what they did is sneaky and would completely break my trust with them.