What significance do the items Boo Radley leaves for the children have ?

All of the items are related to the children. Boo knows that each or both children will appreciate them. 

Here is the list:

2 "pieces of chewing gum" (Scout removed the wrapper from the gun when she removed it from the tree, Wrigley's Double Mint)

2 "scrubbed and polished pennies, one on top of the other" (confirmed to be a 1906 and 1900, both Indian-heads by Jem)

1 "ball of gray twine"

2 "almost perfect miniatures of two children" (Scout and Jem)

1 "whole package of chewing gum"

1 "tarnished medal" (confirmed to be a Spelling Bee medal from before the Finch children were born)

1 "pocket patch that wouldn't run, on a chain with an aluminum knife" (for Jem)

Boo is obviously trying to reach out to the children. He uses what he has to extend gifts. Boo also mends Jem's pants when he got them caught running away. Of course, Boo becomes their final guardian angel when he saves them from Mr.Ewell's attack.