My rule is that people cannot pick up their children until after 3pm so that the children's naptime is protected but I prefer parents to pickup after 3:30 because it's such a busy time for me with diaper changes and snacktime at 3ish. I agree with jazmic that you should tell the Mom that it is too difficult to have her coming at any other time than the one you arrange with her because you are too busy and you really need for her to respect that rule.