It's probable that the residents of Summerville -- which sounds Sumerian, and clearly used to be a Shandor Mining town -- probably don't want to talk about the exterminators that blew up Ivo Shandor's building and thwarted the thing they worship. That would explain why the kids in that town don't know about the Ghostbusters. Consider how Paul Rudd sounds surprised that they don't know what a ghost trap is.
As for Grace McKenna not knowing... we know she's related to Egon as a grandfather, but we don't know who were father is. Maybe her father is someone that was traumatized by one of the climactic events of the first two films. I mean... as a thought experiment, if Egon was a surrogate father to Oscar, that would make perfect sense. His mother was the puppet of demigods twice in five years, and he himself was nearly the vessel for one. That yet unnamed father could move to Chicago and away from the center of supernatural activity in New York and likely wouldn't want to talk to his young children about it.
For instance, I had a grandfather that fought in the Korean Conflict. He never talked about it, and when I learned about the conflict forty years after the fact, it was basically a paragraph and a dot on a timeline. The conflict itself was a way to discuss MacArthur getting fired without elaborating on the hell troops experienced there. I knew the conflict happened, but it would take me actually reading about it (at the tender age of 35) to fully understand it. I sure as hell wouldn't have known what an artifact from that conflict looked like if I stumbled on it, and I would have needed someone older to explain its significance to me.
Add that all up, and it's possible that kids only know about the Crossrip as an event name and date for a quiz, or that Summerville lives in a state of denial about that event.