Why are the children fascinated with the Radley house? What was the first "dare" made concerning the house?

The children are fascinated with the Radley house because of the stories, myths, and superstitions that surround the man inside. 

"The Radley Place fascinated Dill. In spite of our warnings and explanation it drew him as the moon draws water, but drew him no nearer than the light-pole on the corner, a safe distance from the Radley gate. There he would stand, his arm around the at pole, staring and wondering" (8).

Jem and Scout...

The children are fascinated with the Radley house because of the stories, myths, and superstitions that surround the man inside. 

"The Radley Place fascinated Dill. In spite of our warnings and explanation it drew him as the moon draws water, but drew him no nearer than the light-pole on the corner, a safe distance from the Radley gate. There he would stand, his arm around the at pole, staring and wondering" (8).

Jem and Scout fill in the newcomer, Dill, with any and all kinds of information that they have about Arthur (Boo) Radley.They tell him about the time when he was a young man and got caught up with a bad crowd of Cunninghams. The boys were joyriding backwards in the square and got brought up on charges before the judge. Everyone was sentenced to go to an industrial state school, but Mr. Radley wouldn't have it. He promised the judge that nothing like that would happen again and Boo was shut up for life by his father.

The people of Maycomb started believing that Boo roamed the streets at night, no thanks to the gossiping of Miss Stephanie Crawford. From then on, people thought that anything that went wrong in town was because of Boo. Children won't eat the nuts that fall from the Radley yard into the schoolyard. People won't pass the Radley house at night, and even Calpurnia said the following one time:

"'There goes the meanest man ever God blew breath into,' and she spat meditatively into the yard" (12).

Calpurnia's spitting is an act of superstition to protect her from the evil vibes coming off of Mr. Radley as he passed by her. Hence, the stories, the myths, and the superstitions surrounding the Radley home not only captures the children's imagination, but that of the whole town of Maycomb as well.