The robbery happened at 2pm on Friday (November 30) at Queen Elizabeth Gardens in the town centre.
The boys were approached by two Asian teenagers who demanded money and assaulted two of the victims.
The robbers then made off with a quantity of cash.
The first suspect is described as Asian, of a slim build, around 5ft 9ins tall and aged in his late teens.
Both suspects were wearing dark coloured clothing.
Anyone with information on the incident is urged to contact DC Ian Carruthers using 101, quoting log 13180601193 or use the live chat facility on the West Yorkshire Police website.
Information can also be given anonymously to the independent Crimestoppers charity on 0800 555 111.