Children as young as nine are carrying knives in London, according to Sadiq Khan.
London's mayor said young people were being groomed to join criminal gangs and parents had a role to play in tackling the problem.
Mr Khan was speaking after 14-year-old Jayden Moodie was murdered on Tuesday in an area of east London notorious for drug dealing.
Police said they believe the attack was "targeted and intent on lethal force from the outset".
Chief Superintendent Richard Tucker told reporters he could not sleep for thinking about what had happened to such a young boy.
He said of the boy's age: "I think that will strike a chord with so many people and so many parents across the UK."
The teenager, who lived in the area with his mother, was knocked off the moped by a black Mercedes in Bickley Road, Leyton, and then stabbed several times by three attackers as he lay unconscious in the road.