What unites the children pictured above is not their favourite football team or toy, it is the fact they have been abducted by a parent.

And what unites their left-behind parents is that their lives have been turned upside down - their universe now revolves around getting their children back.

They stand together on a sunny Tuesday morning at the National Theatre in London, where they have gathered to mark International Missing Children's Day.

The focus this year was parental abduction, and the day was also used to re-launch the UK's Child Rescue Alert, a system which notifies the public of a child abduction via the media within the first few crucial hours.

It was easy to spot the parents - they were the red-eyed ones, dabbing away the tears as they relived their nightmares.

The most recognisable face at the event by far was Kate McCann's, whose daughter Madeleine disappeared from their holiday apartment in Portugal in 2007.

But the bike-wheeling Australian Ken Thompson also stood out, as did the smiling face of his son Andrew, emblazoned across his lycra outfit.

'Completely disempowered'

Andrew was just three when his mother Melinda took him aboard a flight to Frankfurt with no intention of returning home. It is thought they may be in the UK, or possibly Germany or Spain.

Mr Thompson last saw his son on the 22 April 2008 - two days before he was abducted.

His wife had made unfounded allegations against him and during their family court wrangling, she saw a psychiatric report which concluded she was mentally unwell.

It's up to me to prove where Andrew is...

On the day he realised they had gone, Mr Thompson - a former deputy fire commissioner in New South Wales - went from being powerful to powerless.

"I felt completely disempowered. I was put in touch with an international lawyer who told me to give it three to six weeks but I knew my wife did not fit the profile of a parent who abducts a child," he said.

"Normally they want to go back to the country they came from but my wife was born in Australia."

He said it took months for the Australian authorities to spring into action, and although he has "tried everything", from Interpol to Facebook, it appears his wife and son have "completely vanished off the face of the earth".

So Mr Thompson left his job and is cycling around the UK and Europe to raise awareness of his son.

"One of the most frustrating things is that the media is keen to talk about a child that has been abducted from their country but not one that might have been taken to their country," he said.

He believes in the six degrees of separation theory and that "someone, somewhere knows something".

Mental picture

Mr Thompson talked about how multicultural marriages, ineffective legislation, and cheap airfares had all impacted on parental abduction.

But he said the key was the 1980 Hague convention on international child abduction, which aimed to ensure children's safe return to the country where they normally lived.

Under the agreement, issues of residence and contact are then decided by the courts of that country.

But Mr Thompson said it had fallen behind the times.

"The borders are a lot looser than they were 30 years ago," he said.

"It's up to me to prove where Andrew is and if you can't, the people who administer the Hague convention find it hard to help."

The Hague is on his cycle route and while there, he will deliver a letter on behalf of all left-behind parents.

What keeps the pedals turning for Mr Thompson is a mental picture of Andrew with a huge smile of his face, saying: "I want to come and live with you".

'Trauma'

Blessing Mlandeli knows all about slow, protracted legal processes. The 36-year-old's son Tevin, aged just two, was taken by his father to Zimbabwe in July 2007.

"Even when it had sunk in that he had left the country, I still thought I would have him back in a month," said Ms Mlandeli.

You have to carry on. You have to fight. No-one has the right to take away your child Sarah Taylor

I didn't realise how complicated it was going to be. It took a long while for me to realise that if I wanted him back, it was going to take a big fight."

In addition to the usual red tape, her case is complicated by immigration issues and relations between the UK and Zimbabwe, which has not signed up to the Hague convention.

Ms Mlandeli, a support worker, did not have any contact with her son for a year but she now has custody, and he is living with her sister in Zimbabwe's capital Harare.

She hopes he will soon have the relevant documentation to travel back to the UK.

While her son is in good physical health, she said his education had suffered from the "trauma".

"He cannot read and cannot write properly and his language skills are underdeveloped," she said. "He was taken away at the age of two and that's a crucial time."

Face of hope

Other left-behind parents at the event included Dr Yusra Abo Hamed and Ken Spooner.

Dr Abo Hamed's two young sons - Sami and Rami - were taken to Syria by their father in December 2008, and Mr Spooner's two boys are in Zambia with their mother.

His partner took Devlan and Caelan on a holiday to visit her parents but never returned.

Mr Spooner thought his sons were going on holiday to Zambia

The kitchen fitter said getting his sons back was now "his life" and he was lucky he had savings to fund his legal battles and travel costs.

"I'm very bitter but I know she's my children's mother and I would like to think there will be a time that my children will be back home and they can enjoy both parents," he said.

Nadia Fawzi was a smiling ray of hope for the parents at the National Theatre.

The six-year-old was abducted by her father when she was four and taken to Libya.

Her mother Sarah Taylor moved to the North African country and eventually won custody, but it took involvement at the highest level from Gordon Brown and the Libyan leader Colonel Gaddafi.

Ms Taylor reassured the left-behind parents that their children would never forget them.

"It took two and a half years for me to get my hands on my daughter again. When it's your child you find the strength from somewhere," she said.

"You have to carry on. You have to fight. No-one has the right to take away your child."