In the beginning of To Kill a Mockingbird, who gives Scout and Jem the idea of making Boo Radley come out?

Simply put, Dill gives them the idea. After the kids meet
Dill and summer seems to begin running its course, the kids find many things to do. They
re-enact scenes from their favorite stories, they play in the treehouse, and they have a
great time. But as with all children, summer gets boring after a while. Then, the need
for a new adventure arises. The kids began discussing all of the neighborhood legend
associated with Boo Radley, this perked Dill's great
interest:

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“Let’s try to make him come out,” said Dill. “I’d
like to see what he looks like.” Jem said if Dill wanted to get himself killed, all he
had to do was go up and knock on the front door.
(13)

 Since Jem never
declined a dare, this was one that Jem mused over for three days, but he eventually did
go touch the house.