At the end of The Last Jedi:
Some slave children on Canto Bight (presumably) are re-enacting the "battle" on Crait, using crude hand-made figures and vehicles.
My question is, how could the children have known about that battle and what happened during it? Also, is there any indication as to how much time has elapsed between that battle and the re-enactment by the children?
This was explained in the official tie-in book "Star Wars The Last Jedi The Visual Dictionary"
The boy who uses the Force to control the broom is Temiri Blagg. The girl is Arashell Sar and the other boy is Oniho Zaya.
The Last Jedi The Visual Dictionary describes these stable hands as "children abandoned on Cantonica by losing gamblers" who "engage in imaginative play to fill their downtime. Travelers from distant worlds bring them fragmented tales of adventure that excite their young imaginations." The last stand of Luke Skywalker is now one such legend ...
On a personal note, I think since the boy is Force sensitive he may have seen part of all of the battle in a vision.
No, there's no indication of how much time passed between the battle and the children telling the story.
It's possible that any of the First Order soldiers told the story to a third-party, and that third-party's story evolved throughout the galaxy. Someone surviving a ton of blasters from AT-ATs is pretty significant, of course it would be come a legend.
