Indy you've already been given very good advice so I haven't any to offer. I just want to say bravo to you for seeing an opportunity of really impacting this girls horsemanship. Yes, kids are not always the best of reading or judging a situation, but I bet she will be all ears and whatever you say will be magnified to her because of the experience.
Glad to hear everyone concerned (including the horses) are all right, and the girls parents are reasonable and sensible people.
Be the kind of woman that when your feet hit the floor each morning the devil says, "Oh crap, she's up!".