A 13-year-old boy had a Rambo-style knife held to his throat as a pair of moped muggers robbed his father's Rolex watch.
The two crooks, one riding the moped and the other pillion, committed the callous attack in London's West End, police said.
They pulled up beside the victim's car in Duke Street on Saturday afternoon, when the pillion rider brandished the gruesome knife and held it to the boy's throat, as they demanded the man's possessions.
The father, 42, complied and the pair took off along Brown Hart Gardens, towards Park Lane, with a Richard Mille Watch, a Rolex watch and a diamond ring.
Detective sergeant Andy Swindells, of the Westminster robbery squad, said: "This was a terrifying robbery where the suspects threatened serious violence and held a knife to the victim's young son. Several items were stolen which had high sentimental value for the victim."
He appealed for anyone with information or witnesses to the attack, which happened at around 5.35pm, to contact the authorities.
"This happened just off a busy high street, at a time when there would have been lots of people in and around the West End," Mr Swindells, who is leading the investigation, said.
"It may be that you were saw the moped make off immediately after the robbery, or you were driving through central London at this time and might have dashcam footage that could assist the investigation. We would urge you to get in touch right away."
Both suspects were helmet and spoke with local accents. The pillion passenger, who brandished the knife was said to be of stocky build, aged in his 20s or early 30s. He was wearing a puffer jacket.
No arrests have been made and the investigation is ongoing.
The Westminster Robbery Squad can be called on-call mobile on 07776244676 quoting CAD 005934/04JAN20 or people can call 101. Alternatively they can contact Crimestoppers on 0800 555 111.