What reason do Jem and Dill give for trying to peek into the Radley window on that particular night in To Kill a Mockingbird?

Jem and Dill peek in the Radley window to get a look at Boo Radley.

Boo Radley is the neighborhood monster.  All of the children are afraid of him, and even the adults spread rumors about him.  He has stayed in his house since he was a teenager, and he never comes out.  Naturally he becomes a boogeyman for the neighborhood.

Inside the house lived a malevolent phantom. People said he existed, but Jem and I...

Jem and Dill peek in the Radley window to get a look at Boo Radley.

Boo Radley is the neighborhood monster.  All of the children are afraid of him, and even the adults spread rumors about him.  He has stayed in his house since he was a teenager, and he never comes out.  Naturally he becomes a boogeyman for the neighborhood.

Inside the house lived a malevolent phantom. People said he existed, but Jem and I had never seen him. People said he went out at night when the moon was down, and peeped in windows. (Ch. 1)

When Dill joins Scout and Jem in Maycomb, he is fascinated by Boo Radley.  The children’s new game becomes to try to get Boo Radley to come out. Although they make up all kinds of stories about him and reenact his history in the yard, the children are really just curious.  Atticus tells them to stay away from Boo Radley.

Dill says that they are going to leave a note for Boo Radley offering him ice cream.  He thinks that if he comes out to sit a spell he’ll feel better.  Scout does not like the whole plan.  She wants nothing to do with Boo Radley.  They explain to her that this night is perfect.

Because nobody could see them at night, because Atticus would be so deep in a book he wouldn’t hear the Kingdom coming, because if Boo Radley killed them they’d miss school instead of vacation, and because it was easier to see inside a dark house in the dark than in the daytime, did I understand? (Ch. 6)

Unfortunately, Jem gets scared off and loses his pants.  Scout does not want him to go back for them because Nathan Radley might shoot him, but he says he has to. He does not want Atticus to find out that they disobeyed him, because Atticus would be disappointed in him.