Atticus certainly wasn't expecting his children to show up at the jail on the same night he was standing guard over Tom Robinson. He knew that others might arrive intent on harming Tom, and sure enough, a group of cars soon "stopped in front of the jail." It was a group of men--a lynch mob--who planned to take Tom and hang him. When Jem, Scout and Dill suddenly appeared to say hello to Atticus,
I thought he would have a fine surprise, but his face killed my joy. A flash of plain fear was going out of his eyes, but returned when Dill and Jem wriggled into the light.
Atticus knew that it was a dangerous situation, and although he was willing to face the men alone--and perhaps suffer a beating or worse from the men--he did not want his children involved. He ordered the children to leave, but Jem "shook his head." When one of the lynch mob grabbed Jem, Scout came to his rescue.
I kicked the man swiftly. Barefooted, I was surprised to see him fall back in real pain. I intended to kick his shin, but aimed too high.
It was the children's presence which saved the day for Atticus and Tom. Scout's innocent repartee with Mr. Cunningham shamed him into ordering his accomplices to call off their mission, and the men soon left Atticus and the children alone at the jail--along with the unseen newspaper editor B. B. Underwood and his shotgun standing guard above.
Atticus didn't reprimand Jem because of the bravery he showed in sticking by his father, and because their appearance is all that saved Tom from being murdered.
Atticus and Jem were well ahead of us, and I assumed Atticus was giving him hell for not going home, but I was wrong. As they passed under a streetlight, Atticus reached out and massaged Jem's hair, his one gesture of affection.