Like most children, Scout and Jem are quite superstitious. Theirs is an enchanted world in which all manner of strange, inexplicable phenomena regularly occur. In their childish worldview, even ordinary objects like old Indian-head pennies can be invested with magical properties—Jem certainly seems to think so. He believes that the coins found stashed in the knothole of the Radleys's tree will bring good luck. At this point in the story, Boo Radley is still very much an urban legend to the Finch children—a mysterious, scary figure. It's understandable, then, why Jem and Scout would see the old pennies as having strong magical powers. It makes perfect sense to them that anything touched by the mysterious Boo must have something unusual about it.

Jem and Scout are surprised to find goods in the knothole of a tree. They are even more surprised to find Indian head pennies. In their mind these Indian head pennies are more important and valuable than the gum that they found before. 

According to their logic, Indian head pennies are powerful magic, because they come from Indians. There is no further explanation. The assumption that the reader is left with is that within the thought world of Jem and Scout, Indians possess magical powers. So, their pennies are powerful as well. 

Scout and Jem believed that these pennies would confer good luck such as a long life, good health, and doing well in school. The words of Jem and Scout will give you the context and their worldview.

“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. 

To Kill A Mockingbird