The incident happened at 5.45pm on Wednesday, October 4, when the six-strong group, all aged between 17 and 18, confronted the 13-year-old victims on Blackwood Road in Streetly.
Both models were Carrera Sulcates worth hundreds of pounds. One was grey and bronze and the other was blue with a quick release seat and new handle bar grips.
Two of the suspects were mixed race. One was 6ft 2ins tall, clean shaven with short dreadlocks and was wearing a black hooded top, black tracksuit trousers with a white stripe along the seams.
The other was 5ft 9ins tall, clean shaven, wearing a black hooded top and jogging trousers.
Investigator Claire Kidd, from West Midlands Police Force CID in Bloxwich, said: “Two of the group were on foot and four were riding bikes that appeared too small for them – it’s possible they may also have been stolen from children so we are looking at whether we’ve had similar theft reports.
“One of the group produced a knife and told the boys to get off their bikes and run; fearful for their own safety they gave up their bikes and ran away.
“These are really cowardly attacks on young children by much older teenage bullies. We need to know who these attackers are and I would urge anyone who suspects who might be involved, or who has seen bikes matching the description of those stolen, to contact me so we can take action.”