The weigh-in for Billy Joe Saunders' fight with Willy Monroe Jr was interrupted on Friday when the Briton's young son attacked the title challenger on stage.
As the two fighters posed for photographs, Stevie Saunders moved between them before punching and kicking Monroe Jr below the belt.
The American had ruffled the young boy's hair but his friendliness was not reciprocated by the younger Saunders, who had to be dragged away by security after the attack.
"My son's been taught when a stranger puts their hands on them they don't know punch and run away: self defence," Saunders tweeted afterwards.
Both fighters had just weighed in at 11st 6lbs ahead of Saturday's showdown. They were flexing their muscles in front of the assembled media when Stevie attacked Monroe Jr, who seemed more shocked than hurt - despite being hit in a sensitive area.
The disrupted weigh-in caps off an eventful fight-week for the two boxers, who were involved in a fracas of their own on Wednesday night.
During a media workout at London's Peacock Gym, the two fighters came head-to-head before Saunders launched a homophobic slur at his opponent.
The Briton labelled him a "queer" and told the American's trainer, T.J. Nolan: "Next time bring your mum... we'll f*** her like a b****."
Saunders is the favourite to retain his WBO belt when he takes on Monroe Jr at the Copper Box Arena on Saturday night.
Victory would help keep alive his hopes of a unification bout with the winner of Saul 'Canelo' Alvarez and Gennady Golovkin.
It's not the first time Saunders' kids have hit the headlines in recent months. The WBO titlist came in for criticism in June after he let Stevie drive a 4x4 car around a field.
Saturday's clash with Monroe tops a packed bill of boxing at the Copper Box Arena, which features a host of young British talent.
Unbeaten duo Daniel Dubois and Anthony Yarde will look to enhance their burgeoning reputations with victories in London.
Dubois can pick up the second belt of his career when he takes on AJ Carter for the vacant Southern Area title in only his fifth fight.
Yarde, meanwhile, can add the vacant WBO Inter-Continental light heavyweight strap to his European belt with victory over Norbert Nemesapati.