The incident happened at Longton recreational ground, off School Lane, around 6pm on Thursday.
Two teenage boys approached a group of young people playing in the area and demanded two of the lads hand over their bicycles.
The dad of one of the youngsters said: "They said, 'If you don't give us your bikes, we will stab you.'
"It was a threat they took seriously and were scared by. It seems to be happening a lot in South Ribble at the moment - it's the third incident in the last month where one of their friends has had a bike stolen.
"Two others had theirs taken in Penwortham. Another was approached but managed to fight them off and ride off."
One of the bikes taken in Longton on Thursday was a blue Boardman hybrid bike.
The incident has been reported to Lancashire police. Call them on 101 with any information.