“I just happened to be on my own, bigger kids came along and it was the same feeling.” The episode fuelled a desire to learn self-defence and he remembered thinking “I have got to learn karate and be a black belt, and then I’ll get them”.“It was the worst thing,” he said.Sir Paul has written a children’s book for his grandchildren called Hey, Grandude!, based on the nickname they have given him.He said that, despite his concerns about the threat of street crime, he did not believe children should be “mollycoddled”. “It can be dangerous because if you are in a situation where you need to react quickly you’re not ready for it,” he said. “He was saying the worst thing was that he should have just thumped the guy; he came back and felt a coward. “I said: ‘No, no, no, no! The guy had a knife and you don’t know, the guy might be able to use that knife’. So it is scary these days.”The situation in cities, he said, was “dangerous”, but the young people he knew seemed to “get on with it” despite the tragedies. Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings. Sir Paul McCartney has revealed one of his grandsons was robbed at knifepoint in London, as he raised concern about the “scary” threats facing young people in Britain. The former Beatle said one of his “older” grandchildren had his phone stolen in the mugging, which brought back memories of his own childhood in Liverpool. It comes as the scourge of knife crime shows little sign of abating in the capital, with police and the Mayor of London under intense pressure to bring the epidemic under control.The 77-year-old musician, who has eight grandchildren aged between seven and 20, told The Times: “In London, one of my grandkids, one of my older grandkids, was mugged and got his phone taken. “That takes me back to my childhood when I was mugged in Liverpool, so I am able to talk to him. He added that each generation “has their own set of dangers”, but he tried to hope for the best by showing them “kindness and love”. Reflecting on his mugging on the streets of Merseyside, the singer said: “When I was a kid, it was four guys and they nicked my watch. I was of a similar age.