The incident happened at around 7pm on Saturday, 18th June.
Two boys were on the basketball court in Victoria Park when they were threatened by a group of other boys aged around fourteen or fifteen.
One of the teenagers had a pocketknife. He is described as white, 5ft 8 inchs tall and wore a facemask and black clothing.
The victims ran away and were not physically harmed.
Anyone with information is asked to call 101 and provide the call handler with the reference: 5222149091.