Where do the children store their gifts?

Jem stores their treasure in his trunk.

“Well, Indian-heads—well, they come from the Indians. They’re real strong magic, they make you have good luck. Not like fried chicken when you’re not lookin‘ for it, but things like long life ’n‘ good health, ’n‘ passin’ six-weeks tests... these are real valuable to somebody. I’m gonna put em in my trunk.”