Teens' injuries allegedly tell story of grocer's death

Injuries a group of teenagers sustained after allegedly assaulting a good Samaritan will be used to identify their roles in his death.

Grocer Van Viet Truong, 60, died from serious head injuries after he was allegedly set upon by a group of teenagers while trying to help a fellow retailer under attack in Melbourne's west.

One boy, 15, is charged with Mr Truong's manslaughter while another boy, 12, and two girls, both 14, face lesser charges over the August 1 incident in Sunshine.

A children's court magistrate on Tuesday granted detectives ongoing access to photo evidence and hand swab samples taken at the time of the three eldest juveniles - on the basis it could confirm or disprove each teen's involvement.

The youngest boy however, contested the application for legal reasons.

Police argued that investigators could cross-check the material with CCTV and witness accounts to determine the alleged roles and responsibilities of each accused in the incident.

The older boy had red markings and cuts on his knuckles, detective senior constable Cassandra Syddal told the court.

One of the girls had minor bruising and redness on one knuckle while the other had faint red markings and discolouration on one hand.

The alleged offending "was physical assault - hand-on-hand, body-on-body contact," the officer said.

"It goes for one minute, is quite chaotic, with witnesses trying to describe a flurry of activity in one fight," she said.

The younger boy, who had a laceration further up his arm, was the only one of the charged quartet to contest the forensic procedure application.

His lawyer argued his young age meant he might not have known his actions were seriously wrong.

Doli incapax places a high bar on prosecutors proving children under 14 knew the wrongfulness of their actions.

The group allegedly stole items and were chased by a worker out of a clothing shop near Mr Truong's Hong Hung grocery store.