In a licensed Centre at a certain point, usually at the one hour mark I was to start calling social services or the police, if I could not reach emergency contacts to come pick the child up. The child is conidered abadonned. When you had this conversation with her in the driveway and she brought up termination, why didn't you just tell her that if she is late one more time that you would terminate her immediately without notice. Do you charge late fees ?
How does your family feel about this families lateness ? My husband would have had me terminate them, unless there is an accident or a terrible snow storm the daycar children must be gone by 5, when he gets home, it was a part of the deal.
Just remember people treat you how you let them . I would say enough is enough, it is a reasonable expectation that the children will go home on time.
Start running ads and fill those spots and start over....