His chubby toddler face, framed by blonde angelic curls, stares out from the faded family photograph. Next to it the computer-aged image of a man in his late 30s, shows what the youngster would look like now. More than three decades have passed between the two pictures. Almost half a lifetime.

Sandy Davidson was three when he went missing near his home in Irvine, Ayrshire, on the morning of April 23, 1976.

The boy and his younger sister Donna were playing with their Afghan hound Kissie in the front garden. Their grandmother, who was looking after the children that day, had told them not to leave her line of sight. Then, momentarily distracted, she turned away from the window. A few seconds. That was all it took.

The dog ran out of the gate and Sandy followed, Donna toddling behind. What happens next remains a mystery. The dog and Donna returned, but Sandy did not. He never came back. His image remains on the Missingkids website, a poignant reminder of a young Scottish boy who vanished without trace.

A child goes missing in the UK every five minutes, according to child welfare charity Parents and Abducted Children Together (Pact), which today launches a "forget-me-not" campaign to help bring missing children home.

The parents of Madeleine McCann have given their support to the initiative and today will join families of other youngsters who have disappeared at an event at South Bank, London, to mark International Missing Children's Day.

Kate and Gerry McCann recently marked the second anniversary of their daughter Madeleine going missing from their holiday flat in Praia da Luz, southern Portugal, on May 3, 2007, just days before her fourth birthday.

Also taking part in today's event will be the mother and sister of Katrice Lee, who was two when she disappeared from a Navy, Army and Air Force Institutes shopping complex in Paderborn, Germany, in 1981. Natasha Lee, Katrice's sister, will make a speech and read a poem.

There will also be speeches from ChildLine founder Esther Rantzen, Chief Constable Peter Neyroud, chief executive of the National Policing Improvement Agency, and Andy McCullough, from Railway Children, the runaway children's charity.

Others who will attend will include police officers from Hampshire and Lancashire Police involved in the searches for Damien Nettles and Paige Chivers. Damien was 16 when he went missing after a night out with friends in Cowes, Isle of Wight, in 1996, and Paige was 15 when she vanished from Blackpool in 2007.

Balloons featuring the faces of these and other missing children will be displayed at the event, which is hosted by the National Policing Improvement Agency.

Lady Catherine Meyer, Pact's founder, said: "It is vitally important to increase public awareness of the problem of missing children if effective action is to be taken.

"Our ability to tackle the dreadful statistics - some 100,000 children missing every year - is hampered by lack of information.

"From why children go missing in the first place, to what happens to them next, our information is incomplete and co-ordination between the police, non-government organisations and private sector could be much, much better."

Parents and Abducted Children Together has campaigned for full police support for the Missingkids website with age progression technology and the Child Rescue Alert, which increases the possibility of finding a child in the vital first hours after their disappearance.

Lady Meyer, whose husband Sir Christopher Meyer is the chairman of the Press Complaints Commission and a former British ambassador to the United States, founded the charity in 2000. She fought a long-running legal battle after her two sons were abducted by her ex-husband, German physician Hans-Peter Volkmann, at an airport in 1994.

The campaign has been given a boost by Tesco, which will sell forget-me-not ribbons, badges and wrist bands to raise funds and awareness. The supermarket will also display posters of missing children and link its website to www.missingkids.co.uk.

Tesco executive director Lucy Neville-Rolfe said: "Our aim is to help the police and organisations such as Pact in the best way we can - raising visibility of this problem with millions of customers every week." The search goes on

Moira Anderson The 11-year-old went missing in Coatbridge on February 23, 1957 after going on a shopping errand. She made the familiar trip to the shop in Laird Street, near her grandmother's home in Muiryhall Street.

She was last seen boarding a bus at about 5pm. Sandra Brown, daughter of convicted paedophile Alexander Gartshore, is convinced her father murdered Moira. Andrew and Renee MacRae Renee and her three-yearold son were last seen on the afternoon of November 12, 1976, driving south out of Inverness. Later that evening, their car was found burnt out in a lay-by on the A9. Almost every officer in the Highlands took part in a search that continued on and off for a full year. Their disappearance remains Britain's longest running missing person's case. Kenneth Jones The 17-year-old vanished from Glenrothes, Fife, in November 1998. He had been learning to play the cornet, like his father, and the pair had gone to brass band practice together that evening. But, within hours of returning home, the teenager sneaked out, taking a sports bag and some clothes. Police carried out a two-day search of woodland near the family's home, but found no trace. Derek Burns The 20-year-old left his family home in West Lothian, in March 1989, after telling his father, also called Derek, he was going to Edinburgh.

Later, he and his wife, Diane, found their son had gone from Edinburgh to Hemel Hempstead to visit his girlfriend. He had tea and sandwiches with her and her friend then left them to come home. He is still missing. Anyone with information on a missing person should contact the National Policing Improvement Agency Missing Persons Bureau on 0808 100 877.