Two young boys have allegedly been robbed and assaulted during an armed robbery at a suburban park in Melbourne's west.

Police say two boys from Melton, aged nine and 11, were at a park on Reserve Road when they were approached by three other youths about 5.45pm.

One of the three youths allegedly pulled out what police described as an edged weapon, demanding the 11-year-old hand over his phone.

The three offenders then took the phone and allegedly assaulted the boy, who had to be taken to hospital with serious injuries.

A short time later, police made three arrests and recovered the mobile phone.

A 14-year-old boy, an 11-year-old and a 14-year-old girl were on Thursday night assisting investigators with their inquiries.