Officers were told the random attack occurred outside Victoria Police headquarters, in Melbourne's city centre, about 1pm on Tuesday.
Superintendent Zorka Dunstan, who witnessed the incident firsthand, said the boy was struck by a 27-year-old man while walking along Spencer St.
After landing a blow to the boy's upper body, the man continued walking before doubling back and putting his mother in a headlock, she alleged.
The man is believed to have then repeatedly stabbed the 46-year-old woman before bystanders intervened to pull the man away.
"We had a citizen immediately intervene, who jumped onto him and just pulled him off the mum, and then the other two added assistance by grabbing him as well," Supt Dunstan told reporters at the scene on Tuesday afternoon.
"It's heroic ... without their intervention, it could have been a lot worse."
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 violent assault.
"Fortunately, they weren't injured, but obviously they have been impacted, witnessing such (an) incident," she said.
Supt Dunstan said officers were horrified such an attack would occur anywhere, let alone a stone's throw from a police station.
"I'm just grateful that we had people assist immediately," she said.
Police swiftly arrested the 27-year-old man, from Sunshine West, who was taken into custody for interview.
Supt Dunstan said he 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.
"It was completely random," Supt Dunstan said.
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