We all make mistake with client at first and then sometimes they take advantage. It gone on this long because you enabled it but you know that anyway.

They are not going stop taking advantage because it in their interest and also because they have no reason believe you won't allow it. You are only one who can force change. So what can you do?

Put in writing firm reminder of what agreement was. This include drop off time, expected early pick up times, and the full time care. Let them know if they come early and not pick up early there will be fee for working longer than agreement. Draw up contact with firm agreement and with costs for when they stay longer.

You can start calling them when later for pick up than agreement.

You can replace them.

For me, I would line up replacement. I find if someone take advantage, even if I rein in, I less believing they will start behave reasonable.

Just because these long term client with three places, you not have to tolerate until they decide to leave - and they will as soon as they get better deal.

Right now, you losing income because they only coming three days instead of 5 days which they committed to. That hard for loss with just one place but they taking up 3 of your 6 places. Half your spaces are at 60% capacity. You losing with the extended days which not needed when Mom off work or Dad go home to shower before coming for children. You losing on older child taking space on reduced fees because at school part of day.

If you not have a contract with these people - create one. Or find replacement and let them go. No contract mean no only do they not have give notice, but you don't have to either.