"Hi Lynn. Your niece has said some disturbing things to me that I thought you should be aware of." And then frame the rude and mean things as kids trying to get a rise out of grownups, and who need to understand that they can't say these things to people, and if she's saying this here to a stranger she's probably saying it elsewhere, and this could get her in trouble in school for bullying. And then the disturbing things: declaring her wish to deface and destroy other people's private property.

Based on this kid's rudeness, you are justified in not wanting her to talk to you at work, it's a distraction from the actual job. Based on the wish to destroy property, this kid shouldn't be in the office, period.