I make it VERY clear in the interview that I cannot under any circumstance be open past 5:30 because my kids have after school activities and I have errands to run. My fee is 1$ per minute and if they late more than three times they are terminated. I also have in my handbook " please respect my hours as my time with my family is important to me". Haven't had any issues yet.

I think the problem here was that you were too lenient from the get go. Set a time limit and fees and stick to it.