The children - Scout, Jem, and Dill - liked to play act and roll each other in an old tire. When one day, the tire (with Scout in it) went into the Radley property, Jem had a new idea for a game. He thought that a fun game would be to act out the drama that they heard taking place in the Radley household.
According to Scout the playacting got very complicated. She says:
To Kill A Mockingbird
"It was a melancholy little drama, woven from bits and scraps of gossip and neighborhood legend: Mrs. Radley had been beautiful until she married Mr. Radley and lost all her money. She also lost most of her teeth, her hair, and her right forefinger (Dill’s contribution. Boo bit it off one night when he couldn’t find any cats and squirrels to eat.); she sat in the livingroom and cried most of the time, while Boo slowly whittled away all the furniture in the house."
The reason why they stopped was because Atticus was catching on to what they were doing. This made Scout think and she realized that it was not a good thing they were doing. The others agreed. So their enthusiasm dissipated.
The children started maturing and became too old for their childish story games. They were also repetitive in their roles of the story, Scout always playing the minor 'unimportant roles'. They soon became bored of playing the same roles over and over again, with the fact that Jem and Dill started becoming more interested in guns and such. The children thus slowly became less interested in their games.