Every year, 2.5 million babies die before they are one year old because of low birth weight, birth asphyxia or birth trauma. All of these conditions happen because the mother is malnourished or in poor health, and because she and the baby don't get proper medical care at the birth.

The fight for survival doesn't end there. Nearly 10 million children under the age of five die every year - more than 1,000 every hour, according to the U.N. World Health Organisation (WHO). The main causes of death at this age are pneumonia, diarrhoea, malaria, measles and HIV.

Half of child deaths are in Africa, even though only 11 percent of the world's population lives there.

About 20 million children under five years are severely malnourished, which leaves them more vulnerable to illness and early death, WHO points out. Children also suffer the impact when basic health services and are inadequate and trained staff in short supply.

See also Biggest killer diseases and conditions in our Health briefing

Several of the Millennium Development Goals set by world leaders in 2000 relate to children. The targets for the world to reach by 2015 include cutting the under-five mortality rate and infant mortality rate, and increasing the proportion of one-year-olds immunised against measles.

The global AIDS pandemic has had a profound effect on children in many parts of the world. In sub-Saharan Africa, generations of children have grown up alone or in the care of their grandparents, after losing their parents to the virus.

Some 15.2 million children under 18 have lost at least one parent from AIDS, UNAIDS calculated in 2007.

And around 2.3 million children have the virus, according to UNAIDS, nearly 90 percent of them in sub-Saharan Africa. Most HIV-positive children die before their fifth birthday, according to the Global Movement for Children.

Screening and drugs can dramatically cut the risk of transmission from HIV-positive mother to child, but most pregnant women in developing countries do not have access to proper health care, HIV testing facilities or education about the virus.

Treatment for children is costly - $1,000 to $1,500 per year. There are few drugs available which can be safely used on children who need different doses and different forms to adults. Just one in 10 children needing anti-retroviral treatment receives it, UNAIDS said in 2007.

Drug companies have not focused on making medicines for children, partly because the market is small and it is difficult to predict demand.

See our AIDS pandemic briefing

A child dies of hunger every five seconds, the U.N. World Food Programme (WFP) says. And malnutrition contributes to more than half of child deaths, according to the WHO. Hungry children don't have the strength to survive illnesses or study at school.

Most kids are short of food not because of a one-off disaster or crop failure but because of chronic, long-term poverty that means their families cannot afford the food in the local markets.

One in four children in developing countries is underweight, U.N. Children's Fund (UNICEF) says. The worst-affected region is South Asia, where more than 40 per cent of children under five are underweight. Rural children are twice as likely to suffer as children in cities.

Wars tear families and societies apart, and children usually get the roughest deal.

They are often preyed on by armed groups to serve as child soldiers, sex slaves, porters, messengers, spies and cooks.

UNICEF estimated in February 2007 that there were more than 250,000 child soldiers as young as nine. And it said the number of armed groups and forces identified as using children had climbed from 40 in 2006 to 57 in 2007.

But other campaigners on child soldiers shy away from trying to calculate how many there are in the world, saying there just aren't any reliable figures, and many of the places where children fight are simply too dangerous for researchers to spend any length of time.

The director of University of Pittsburgh's Ford Institute for Human Security, Simon Reich, said in April 2008 the true figure was probably between 200,000 and 250,000.

Child soldiers are still reported in Afghanistan, Central African Republic, Chad, Colombia, Democratic Republic of Congo, India, Iraq, Myanmar, Palestinian Territories, Philippines, Somalia, Sri Lanka, Sudan and Thailand, according to the Coalition to Stop the Use of Child Soldiers.

Only a handful of states use children to fight in their regular armed forces. But Human Rights Watch estimates that some 70,000 children make up one-fifth of Myanmar's army. Children also fight in the Karen rebel army that's been at war with the state since 1949.

Some young people join rebel groups freely, others sign up in search of food and security, and many are recruited against their will. When children are forced to fight, they are often given drugs to dull them into submission and get them through the brutality they witness and participate in. And it's common for commanders to put children through initiations where they have to carry out some horrific, bloody task in order to bind them to the group and make them feel it's impossible to return to the society they knew before.

Child soldiers have a tough time becoming civilians again when wars have ended. They're rarely included in demobilisation compensation packages and their families and communities can be reluctant to take them back.

Girls who've had babies after being raped during war or "married" to militia men are often especially stigmatised.

About 45 percent of refugees are under 18, according to the U.N. refugee agency, UNHCR.

Their education often suffers, and many experience emotional problems.

For refugees sheltering in camps, conditions vary widely. Some camps provide relatively good levels of health care and schooling, while others are dangerous places where women and children risk being attacked on their way to the latrines.

Some camps and war zones build up a culture of exploitation, where it becomes normal for children to trade sexual favours for soap, schooling or food. Aid agencies and peacekeeping missions do their best to confront exploitation and stamp it out.

Times of lawlessness and conflict make unsafe environments for children. For example, girls in Darfur risk rape when they leave camps and villages to gather firewood. Children and babies in Central African Republic are preyed on by bandits who kidnap them for ransom.

Children's curiosity can be fatal in war zones where landmines and other explosives litter the ground long after the shooting has stopped.

Civilians generally account for around three-quarters of recorded deaths and injuries from landmines every year, and a little over one-third of civilian casualties are children. Nearly all of them are boys, according to the International Campaign to Ban Landmines.

In some places, the majority of casualties are children. For example, 66 percent of landmine casualties in Somaliland were children in 2006, and likewise 59 percent in Afghanistan and 53 percent in Nepal, according to the Landmine Monitor Report 2007.

Campaigners against the use of cluster bombs argue that children are often drawn to unexploded bomblets, which look like appealing, brightly coloured capsules.

On the other hand, observant children can help demining experts map where dangerous explosives are lying around a community - on farm land, near rivers, by schools - so they can be marked to keep people away until they can be destroyed.

It's devastating for anyone to live through a major disaster like an earthquake, and it can be especially traumatic for children, who often take a long time to get over feeling jumpy and fearful of it happening again.

Risk reduction campaigners have tried to get authorities to prioritise building quake-proof schools, having identified that they - along with hospitals - are often especially vulnerable when logically they should be extra safe.

Psychological experts say that children recover best when their routines are resumed as quickly as possible, so getting back to school after a disaster can play a major healing role.

Educating children on the risks of disasters before they happen can make a massive difference, and often inspires them to stay involved with the issues that affect their communities.

Children can be heartbreakingly cute and profoundly photogenic. No wonder aid agencies tend to use them to showcase campaigns and appeals, and journalists often hone in on the "personal story" to make a humanitarian crisis come alive.

But a whole minefield of ethical problems opens up when aid agencies and journalists put the spotlight on children.

For aid agencies, the quandary is how to tug donors' heartstrings and boost the organisation's profile without breaking self-imposed rules about portraying people with dignity.

The days of fundraising pictures of starving black babies silently holding out their hands for help are long past for most agencies. But they struggle to use images that show communities acting to change their circumstances, rather than the helpless victims' look.

Newspapers often select pictures from Africa using totally different standards to those that would normally apply, say aid workers. For example, picture editors would usually think at least three times before publishing photographs of naked children, unless they are African famine victims.

As a journalist, it can be hard to know when and how to interview children in a responsible way as well as one that will encourage them to speak their minds and make interesting comments.

But it's not impossible. Aid workers and journalists have come up with tips for helping reporters interview children effectively and responsibly.

They say you need to think about getting permission for talking to children and using their pictures, consider whether or not it's appropriate to identify children by their full names, and always explain what you're doing without raising false expectations.

AlertNet has some guidelines on how to report on children in crises, and Save the Children has published a booklet on interviewing children.