I have my suspicions that this may be mom-driven, as much as it is child-driven.
I think you give one more warning.
"It is important that you are here by 9:30 because....". After that, you go about your routine and don't answer the door past 9:35. You cannot sit around waiting on her/him.
This will only happen once, I think!
How is she showing up a half hour late to work, consistently?? I have never had a job that was ok with that!! LoL