On a cloudless Sunday morning a few miles outside Sierra Leone’s capital, Freetown, a gang of teenage boys is enjoying a noisy kick-about on a dusty pitch when a young white man with a shaven head runs down a dirt track to join them. He kicks the football and the boys nearest him stop in their tracks.

“Is it him? It is! It’s David Beckham!” they shriek. “David Beckham! He’s here. With us!”

Beckham smiles his famous dazzling smile; the young Africans whoop with joy and, as the news is shouted around the pitch, the boys run over to touch his head and stroke his tattooed arms. He soon peels off his T-shirt to play in the heat.

But he wasn’t in Sierra Leone last week to play football. He was visiting the world’s poorest country as a goodwill ambassador for Unicef to remind the world of the shocking child mortality rates in developing countries.

When Unicef invited him to go on his first field trip, he said, “Show me the worst you’ve got.” Sierra Leone has topped child mortality rates for several years.

Beckham thinks he can make use of his fame to focus attention on what’s happening in the poorest part of Africa. “I’m a footballer,” he reasons. “A lot of people are aware of what I’m doing and they listen to what I say. I want to make awareness of what Unicef is doing bigger. And I’ve always thought there was something more for me to do with my life than just play football.”

After about 20 minutes of heated if not startlingly brilliant football, the game descends into horseplay, and two minders who’ve been observing Beckham from a discreet distance help the star extricate himself.

One, called George, a former SAS man wearing mirror sunglasses, says he does “a lot of work with the Beckhams”. He has a deep cut close to an eye and a healing broken nose as testament that his job can get tricky.

“Mad!” beams Beckham, back in the convoy of vehicles taking a circuitous route into Freetown to dodge his more exuberant fans. He was mobbed on arrival in a privately chartered jet at Freetown’s airport the previous day.

“Frankly, that was the worst I’ve ever experienced,” he says. “They had their hands in my pockets, my mouth. I wasn’t frightened, but it wasn’t what I expected.”

Even Unicef’s Sierra Leone media head, Alison Parker, was not told the identity of the visitor until the 11th hour, although she’d guessed. “Tony Blair and Prince Charles had already visited,” she said. “I knew it wasn’t the Queen or Gordon Brown, so it had to be David Beckham.”

The football star is evidently excited at his debut trip. At dinner, relaxed and with a beer in his hand – and making sure you have one too – he is chatty, engaged and almost excessively good-mannered. On his first visit to the country where more than a quarter of the children die before their fifth birthday from easily preventable diseases such as measles and malaria, he says he expects to get upset.

He sometimes seems ill at ease with Unicef’s fact-packed videoed messages, but when it comes to the nitty-gritty of meeting the world’s poorest and most vulnerable children and their mums he handles each situation with grace and aplomb.

On the first day, after a predawn start, when Beckham doesn’t even have a cup of tea, we drive to a feeding centre in Makeni – once a rebel stronghold where the population includes amputees from Sierra Leone’s 12-year civil war. It was infamous for victims being “invited” to have their arms hacked off at different lengths.

Foday Turay, 5, stick-thin, walks up to Beckham, clutches his hand and shyly presents him with a tiny bunch of yellow flowers. The footballer is visibly moved. Later he says: “I’ll never forget the way he held on to me and wouldn’t let me go the whole time I was there.”

Next is five-year-old Senyo, whose mother has so many other children, she couldn’t afford to feed her properly. The child’s legs became so severely wasted that she could no longer walk. Today, as the little girl teeters towards him, Beckham’s eyes film over. He blinks before he cheers her on, “Good girl; strong girl! Well done!”

Back at the hotel he admits: “I was close to tears. I had to step back and take a deep breath. I am a very emotional person, and I’m even more emotional now that I’m a parent. But what stopped me, personally, crying was knowing that what these kids are going through is far, far worse than my feelings.

“I’ve also seen celebrities crying in this situation, and I didn’t want to be seen crying on camera in front of the children. Because sometimes that comes across as natural and sometimes it looks false.”

Does meeting such poor children make Beckham feel guilty about his privilege and wealth?

“I thought I would feel bad,” he admits. “And I did think, how am I going to feel when I get back to the hotel? But it’s all been surprisingly positive because I could see what Unicef is doing to help the children.

“What made the day extra-special was not being recognised,” he adds. “In most countries of the world even children of four and five years old know who I am. But many of the children today hadn’t seen me play football and didn’t know who I was. But they warmed to me without the ‘Oh, that’s David Beckham’ thing. That was nice.

“People ask me what I want to do when I leave football. I’ve never wanted to be a manager. It just doesn’t interest me. I’ve always been passionate about children and although it might sound a cli-ché this is my way of giving something back.”

Beckham’s value to Unicef is obvious: his global appeal. Whether endorsing Adidas footballs or Armani underpants, he is an ad man’s dream. Since he earns a reported £25.6m a year with LA Galaxy, does he really need to make so much from endorsements? Briefly Beckham gets tetchy.

“Five years ago I had 13 sponsors, which was ridiculous, so I’ve made a conscious decision to cut it down to five,” he says. “But people probably don’t real-ise I’m putting a lot of money into the charity I run with my wife – which Victoria’s mum runs so we can know exactly where the money’s going. And I pour money into my football academies, which aren’t just about football.”

Everywhere we go Beckham takes photographs, to help explain to his sons about what they call “the poorly children”. Next time he does a Unicef trip, he says, he’d like to bring his eldest son, Brooklyn. “I wanted to this time but he couldn’t take the time off school. I think it would put life in perspective for him.”

Holding two-day-old Mari-atsu, he says, “I love babies when they’re like this.”

And he clearly does. When he’s given bawling six-month-old twins to hold, one on each arm, the babies stop crying. The staff roar their appreciation: “We’ll give you a job!”

It’s a world away from his own family life, where security is such a permanent feature that the Beckhams have told their children the police park in their driveway so they can enjoy the siren. His boys – Brooklyn, 8, Romeo, 5, and Cruz, 2 – have been the subject of kidnapping threats. “Clearly, our children’s lives aren’t normal,” he says. “Not every child has security guards wherever they go. But we’re trying to bring them up as normally as possible.”

Recently Beckham was distressed to see a child who’d fallen and hurt himself in the p l a y g r o u n d ignore his mother and run to the nanny. “Some people with our sort of lives let their nannies do everything. Of course we have nannies but whatever we’re doing we make sure one of us is at home. We want our children to know we’re their mummy and daddy – and they do. They’re quite like I was as a kid – sensitive but knowing what they want and trying to get it.”

Over the years Beckham has discovered an extra magic to performing well on the football pitch. “It’s where I feel happiest and where I feel safe,” he confides. “In life I feel safe when I know exactly what I’m doing and on the pitch I know I can perform well.”

Whatever anyone thinks of Beckham’s move to Los Angeles, it clearly makes sense for him – part of a career orchestrated with intelligence. Yet an entire web-site is devoted to “thick Beckham” jokes. “In the first years that sort of thing affected me,” he says. “But actually it’s fine if they want to think that of me; it doesn’t bother me any more.

“One of my sayings is that ‘it’s a round world which is always turning’. I’m either at an extreme high or an extreme low. And that’s my career: I’ve been sent off in the World Cup and slaughtered by the media; or I’ve been England captain and heading for 100 caps. I think you come out of the low times best when you show dignity. Silence can speak louder than words.”

And with that, at the end of our second day in the noisy MJ hotel, where the facilities include mosquito nets with holes and a trickle of tepid water, Beckham calls his children and his wife with his latest bulletin on “the poorly children”.

“Have you been a good boy?” he asks each of his children, before adding, “I love you.”

When he says, “The malnour-ished kids I’ve seen – you wouldn’t believe it”, I imagine he is speaking to his wife.

“We can’t turn a blind eye to the children dying every day from preventable causes,” he says before he returns to England. “I’d like other people to see what I’ve seen in Sierra Leone; it’s changed me, without a doubt.”

How to help

To make a donation visit www.unicef.org.uk/childsurvival.

Unicef urgently needs funds to help stop children dying before they reach their fifth birthday. £1.50 could immunise 20 children with safe equipment. A few pence can pay for one sachet of oral rehydration salts, which, mixed with safe water, helps children combat dehydration and diarrhoea. Just 30p can buy 50 water purification tablets, each treating 4-5 litres of water, enabling children to drink and wash safely.

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