Continuity mistake: When Walter tells Christine about the fight with a boy at school, Walter's left hand moves from hanging down by his side to fidgeting with his handbag strap. (00:05:00)
Continuity mistake: When the "substitute" boy gets off the train he is with an older women. The reporters ask the mom and boy to take a picture together. You can see from the camera angle that the older woman is in the background when the flashes go off taking the picture. Later in the movie, the psychiatrist holds up a newspaper article with the picture in it. In the newspaper picture, the older woman is not in the background.
Continuity mistake: When the cop shows the killer kid a picture of Walter, the back of the photo keeps changing from wrinkled to straight repeatedly.
Continuity mistake: When Angelina is sitting down at the table at the mental asylum, she is sitting across from a heavyset woman. Before she starts talking to the blond woman, the lady across from her takes a drink. Just as she is raising the glass, the scene cuts to another angle and the lady's glass is down on the table again.
Answer: When the accomplice is originally questioned he says that killer told him a few 'may have escaped', implying he never knew if they were caught.
Also, it's implied his accomplice a young boy himself was scared of ending up like the boys himself, sadly I think it was done for survival.