Originally Posted by Heidi:
When she apologized, did you slip and say "oh, don't worry about it" or something like that? We are always so nice....until we're not. 
I would email or call her and just clarify as the PP said.
...Sorry if I was rude, I was worried about DH, and the late thing is getting excessive, especially on a Friday. You adore her child, and you've always enjoyed working with her, but now you have to scold her or charge her a late fee. You don't want to do that, so can she please make sure to get here by 5:30? Thanks so much...
I tend to be too nice in person with the "oh, don't worry about it" but then steam about it later. You need to be honest and while you may want to apologize for being a little short with her at the door you need to address the late issue now before it becomes a bigger issue.
And I understand what you are saying about her bringing the other child in with her and that speaks volumes about how she sees being late. If she had any regard for it she would have just run in and grabbed dck while the other child waited in the car in your driveway.