Kindergartener said the n word

How would you respond? Today a white kid called the only black kid in the class the n word. They are 5 years old. I was shocked.

The black kid was sitting at another student’s seat so I jokingly said “Oh, wait! You’re not ______”

Then another kid goes “yeah that’s not ____ you’re a n—“

Edit: thanks for responses. Sent an email to admin about the incident.