In Harper Lee's To Kill a Mockingbird, there are several clues that indicate that Boo is watching the children.
When the children are playing the "tire game" and Scout rolls into the Radley yard, she later admits that she heard laughing from inside. Of course, Boo leaves little gifts in the tree on his property for the children to find on their way home from school, until Mr. Radley cements the hole closed. At another point, the children see the blinds move in the house: had it been Mr. Radley, he would have come out to yell at the kids.
During the fire at Miss Maudie's, Scout ends up with a blanket around her shoulders, put there quietly by Boo to keep her warm. Jem's pants are repaired, sewn like a child might mend them, after Jem gets caught on the fence escaping from the Radley's yard.
Finally, it is only by keeping an eye on the children that Boo is able to save them from Bob Ewell when he attacks them at the end of the novel. Boo becomes an unlikely hero in this way.