Because he used to be tormented when he was younger and Atticus heard about it and wanted to try stop Jem and Scout from Boo Radley reliving those moments.
Tell your friend to stop bothering you or tell an adult
Atticus tells Scout very little about the Radley house, as he wants to respect the privacy of the reclusive Radley family. He also wishes to instill in Scout a sense of empathy and understanding for others' perspectives. Atticus encourages Scout to be compassionate and respectful towards Boo Radley and his family.
Jem and Dill get the idea to write a note to Boo Radley asking him to come outside and tell them what he does inside his house. Furthermore, they write that they will buy him an ice cream and will not hurt him. They put the note on the end of a fishing pole and try to attach it to a window shutter on the side of Boo's house.
Atticus visits the Radley house to pay his respects upon the death of Mrs. Radley. Scout and Jem suspect that Boo caused her death and are eager to prove their theory. They are disappointed to learn from Atticus that she died of natural causes. Then Scout asks Atticus if he "saw Mr. Arthur." Atticus sternly replies, "I did not." That is all she hears about it.
Atticus found out about the children's game involving Boo Radley when Heck Tate mentioned it during Tom Robinson's trial. Tate revealed that he had asked Atticus not to tell the children about Mr. Radley's courage in saving them from Bob Ewell, as it might prompt them to reenact the story. This indicated to Atticus that the children had indeed been playing a game about Boo Radley.
Tell them.
Dont be afraid to stand up for your friend. Tell them to stop bothering her, or tell the teacher.
she did not, but later after mrs. Maudie's fire Jem reveals everything
Tell them to get over you :)
Tell him to stop bothering you or you will report him for harassment. Simple.
In chapter 8 Atticus learns about Jem and Scout communicating with Boo Radley through the knothole. Atticus tells the children not to tell anyone that they had seen and been in contact with boo.
pages 70, 71 in the perennial classics version: "As usual, we met Atticus coming home from work that evening. When we were at our steps Jem said, "Atticus, look yonder at that tree, please sir." "What tree, son?" "The one on the corner of the Radley lot comin' home from school." "Yes?" "Is that tree dyin'?" "Why no, son, I don't think so. Look at the leaves, they're all green and full, no brown patches anywhere--" "It ain't even sick?" "That tree's as healthy as you are, Jem. Why?" "Mr. Nathen Radley said it was dyin'." "Well maybe it is. I'm sure Mr. Radley knows more about his trees than we do."