The 10-year-old boy was allegedly struck by a 27-year-old man while walking along Spencer St with his family - outside Victoria Police's headquarters in Melbourne's city centre - about 1pm on Tuesday.
After landing a blow to the boy's upper body, the man continued walking before doubling back and putting his mother in a headlock when she tried to intervene, Superintendent Zorka Dunstan, who witnessed the incident, said.
The man is believed to have then repeatedly stabbed the 46-year-old mother in the upper body with scissors before bystanders intervened to pull the man off her.
"It's heroic ... without their intervention, it could have been a lot worse," Supt Dunstan said.
Police quickly arrested the 27-year-old man and seized a pair of scissors.
The man from Sunshine West, in the city's inner west, was overnight charged with a series of assault-related offences including affray, intentionally causing injury, two counts of unlawful assault, conduct endangering life and conduct endangering serious injury.
He was remanded to appear at Melbourne Magistrates' Court on Wednesday.
The boy and his mother, who were visiting from the Phillip Island township of Cowes, were taken to hospital with non-life-threatening injuries.
Supt Dunstan said the woman's two other children were shaken up after witnessing the attack.
"Fortunately, they weren't injured, but obviously they have been impacted, witnessing such (an) incident," she said.
Supt Dunstan said officers were horrified such a random attack would occur anywhere, let alone outside a police station.
"I'm just grateful that we had people assist immediately," she said.
Supt Dunstan said the man was known to police in Victoria, NSW and Western Australia, but did not have an extensive history.
Police do not believe the parties knew one another.
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