Two boys aged 14 and 15 were approached in Tarring Park, off Church Road, by two suspects at about 5.40pm on Friday, September 14, police said.
One held a knife and demanded money from the victims, but a passer-by intervened and the boys managed to run away unharmed, according to police.
Police describe the first suspect as white, aged 15-16, of stocky build, with very spotty skin.
He wore a light grey zipped hoody with the hood up, a navy blue tracksuit and black Nike Air Force One trainers, said police.
The second suspect is described by police as white, aged 15-16, with square glasses with large lenses.
He wore a cream tracksuit top and navy tracksuit bottoms, and had a navy side square bag, and a gold ring on his right ring finger, police said.
Anyone with any information about the incident, report it online or call 101, quoting serial 1098 of 14/09.
In particular, police would like to hear from the passer-by who intervened in the incident.
Alternatively, contact the independent charity Crimestoppers anonymously on 0800 555 111 or report it online.