A 7-year-old Pittsburgh boy used a folding knife to threaten a 6-year-old playmate so he could take an iPod from the child, police said.
Police aren't identifying or charging the suspect in the July 9 incident.
Information about the incident emerged as WPXI-TV questioned police about another incident on July 12 involving a 6-year-old boy.
That boy, police said, grabbed for an officer's gun when the boy was caught stealing a bike.
Officers are being warned to be more alert to adult-type dangers, even when dealing with younger children, police said.
Police alerted the parents of the iPod robbery suspect and those of his victim and were letting those parties handle the incident, said police spokeswoman Diane Richard.
Children under 10 cannot be charged with crimes in Pennsylvania.