I kinda feel like the idea is these kids might not have even heard the name “Luke Skywalker” until what happened at Crait. This was of course the original plan for Rey, who ultimately just ended up being someone who heard the stories but didn’t totally believe them. For these kids, a full generation removed, it’s certainly possible they’d never heard of the Jedi or what they could do until Luke showed up on Crait and stood up to the First Order, igniting a spark of hope and inspiration in them.
As for how they know, as usual the final scene in Star Wars is ambiguously displaced from the timeline of the rest of the film. Who knows when that was.