I think this is definitely a red flag but I wouldn't give up on them just yet. Sometimes parents who aren't familiar with daycare rules assume that we are happy to spend every waking moment with their child and don't really see that we have a business arrangement and are not simply caring for their children out of the kindness of our hearts. Turn the tables and assert your power and control over the situation. I'm not a fan of just sending out a notice - I would rather speak to the parents in person or on the phone and then re-iterate the conversation on paper to drive the point home. You can just explain that you have obligations outside of work and that your own children deserve time alone with their mom. I would also stress that closing at 6 doesn't mean that they can rush in at 5:59 to pick up their child, especially in the winter when it takes longer to get the child dressed, in the carseat and out of your driveway.
I had to have a very unpleasant conversation like this with a parent who was consistently rushing in 1-2 minutes after I closed. I basically told him what I said above and he has been here 5-10 minutes before closing every pick up. I felt guilty about it for days and questioned whether i was over-reacting, but in the end, his rushing in here just as i close was becoming very stressful for me because i was never sure if he would actually be here on time, which would mean that i woudl have to change my plans for the evening, etc.
Good luck and keep us posted!