I have to pick a scene from The Twenty-One Balloons and retell it using two characters. What scene and characters would be good?
I am going to go another route here, because this is such a gem of a book. Why don't you choose your favorite scene? Stand back, and see which scene you picture. When you think of this book, what scene comes to mind? This is the one you should retell.
The scene in Chapter X, "What Goes Up Must Come Down," in which everyone spends 17 hours over the volcano, might be a good scene to rewrite, choosing, perhaps, one of the parents and one of the children, to focus on how two individuals felt during this experience, with dialogue showing the fears of the child and the adult consoling the child, or with a focus on what a parent might say to a child about having to start a completely new life elsewhere. It must be clear to everyone on the platform that the community's time on Krakatoa is over, and a new life must begin.
Chapter VI, The Gourmet Government, seems like a likely candidate for retelling. It has adventure and vivid, though brief, imagery (du Bois' style embraces simplicity); it has danger; it has several characters to refer to. The two you could focus on would be Mr. F. and the Professor.