Did you reassure her? That's what I would say to do, on the spot, once you realize that a child took you seriously & felt anxious as a result. I think that's pretty much what you can do--acknowledge her worry (how it makes sense that she got nervous having overheard that.) Acknowledge that you said something confusing because you were feeling X, that you shouldn't have said it & you wish you hadn't, and that you will pick her up & would never forget her, even if you are upset. It's as simple as acknowledging you wish you'd said, "I'm so frustrated this morning!"
Just make amends when needed & respond to expressed or evident anxiety whenever you're presented with it. Not in an "Oh, I was just kidding!" way, but in a way that takes her concern seriously, takes responsibility for causing it & gives explicit reassurance. This way, you are modeling something important for her.
You can do it in retrospect, too, if you didn't before.
at 4:43 PM on Nov. 21, 2012