At this part in the novel, Atticus does not know about the friendship that Jem and Scout have formed with Boo Radely. He had known before, about the children being scared of going near the Radley's, but had not known about the incidents that had forged a friendship. Part of the reason that Jem does not want Atticus to reaturn the blanket is due in part to the friendship he has forged with Boo. It is symbolic of their frienship and Boo would understand why Jem did not return the blanket. He also knows that if the blanket is returned to Boo, there is a good chance that there would be trouble caused. Boo cares about both Jem and Scout and returning the blanket could case major problems for both Boo and the children.