Do you have the original agreed upon hours in your contract? If so, pull out those contracts and talk to the families individually about the liberties they have taken. Do you have late fees? My earliest family comes at 7:40 and pick up by 4:10 and my latest clients come from 8:40 to 4:40.

When the Mom who comes for the last child is arriving about 4:50 once in a while I have her son dressed and ready to go and he's helping me water the garden or take out the garbage or waiting while I sweep up, but I let that Mom know that I really don't appreciate it that she's late. The trick is not to allow them to start pushing you. The old saying - if you give an inch, they will take a mile - is sooooo true.