The flashback to the fireworks adds some unexpected hope by showing that people create beauty even amid poverty.
In the story, it is clear that the people who lived in the reservation did not have much money (in fact, at one point the narrator says "who does have money on a reservation, except the cigarette and fireworks salespeople?"). However, for two boys who have nearly nothing, seeing fireworks in the night sky (even though they are to celebrate a holiday that isn't theirs) means everything. It is a beautiful image.
It is clearly stated by the narrator once again: "The fireworks were small, hardly more than a few bottle rockets and a fountain. But it was enough for two Indian boys."
