I can just imagine how you feel and I would've felt just as terrible, if not worse. You already feel bad about it so berating you for something that has happened to a lot of us and some of us even more than once, won't make sense.
So I'll just say that we learn every day and this experience has taught us to return a smile to kid. Make their day somehow, the best way we can. I hope you feel better dear.
I should have returned more than a smile. A wave would have acknowledged her kind deed, and I'm sure would have made a difference