The children have no idea where their treasures come from. They find them in the tree and take them, assuming that the treasures are left there for them but with no real proof of this. However, the gift of two pennies does provide that proof for the reader. Whomever is leaving the items must be clearly targeting the Finch children - the two Finch children, as well as the only two children on the street. This lets readers know that the gift-giver has some knowledge of the children, as if he is watching them and providing things he knows they will enjoy.