Najia Warshagha, age nine, is a veteran of three conflicts in Gaza. She cannot sleep properly and is always terrified

She seemed to represent the collective suffering of Gaza's children: a little girl, eyes cast down, a tear edging beneath her lashes, blood smeared over her forehead, anguish written into her face.

Her picture was taken in the aftermath of the shelling of what was supposed to be a refuge from war, a UN school in Jabaliya. "The world stands disgraced," declared a shocked UN chief after 15 people died and more than 100 were injured.

The girl whose image spread around the world is Najia Warshagha, who at the age of nine is already a veteran of three bloody and devastating conflicts in Gaza. Over four weeks of this war, at least 447 children have been killed and 2,744 injured, according to the UN. Thousands more – Najia among them – are deeply traumatised.

Najia Warshagha was treated for injuries caused by an Israeli strike at a UN school in Jebaliya refugee camp, in Beit Lahiya, Gaza. Photograph: Khalil Hamra/AP

Nine days after the shelling of the school, Najia perches on a sofa at the relative's house where she is staying, a solemn child whose hands twist into tight little balls as she haltingly recalls what happened. "I was in classroom number one, sleeping. There was a huge boom. My mother hugged us, then another missile landed. I was screaming and crying," she says.

Does she still think about it? A pause, then a nod. And, quietly: "I keep dreaming of what happened."

Her mother, Majdolen, 31, fills in the gaps of Najia's spare account. The family left their home in Beit Lahiya, close to the border with Israel, a few days after the bombing began to seek refuge in a nearby school. A few days later, fearing that was also unsafe, they moved to the school in Jabaliya, where around 3,300 people were crammed into classrooms and corridors, spilling out into the school yard. Seven families were sleeping in classroom number one when the missiles struck at around 4.30am. Najia's legs were injured and her four-year-old brother, Ali, was hit in the head. Luckily, neither child's physical injuries were severe. Majdolen also received shrapnel wounds to her shoulder and head. The Jabaliya school was one of six run by the UN that have come under attack over the past few weeks.

The family was taken to the Shifa hospital in Gaza City, a chaotic and overburdened place these past few weeks, whose exhausted staff are working round the clock and where every bed is occupied. Ali spent two days there; Najia was kept in for a week being treated for extreme shock. She is still taking medication. "She is very traumatised since it happened," says Majdolen. "She can't sleep properly, she's always terrified. The children don't want to leave me, they want to sleep with me and they follow me wherever I go."

Despite the August heat, Najia wraps herself tightly in a blanket at night, says her mother. The family is sleeping in its fourth location since the war began but the child is pressing for another move, feeling that nowhere is safe.

Mulling over the wars – Operation Cast Lead in 2008-09, Operation Pillar of Defence in 2012 and the current Operation Protective Edge – concertinaed into her short life, Najia concludes that this one is the worst. On top of the family's desperate search for safety and the shelling of the school, their home has been flattened. "It's gone – there's nothing there," says Majdolen.

Doctors and mental health specialists in Gaza can draw on a bitter store of experience of treating traumatised children. After Operation Cast Lead, a study by the Gaza community mental health programme (GCMHP) found that 75% of children over the age of six were suffering from one or more symptoms of post-traumatic stress disorder, with almost one in 10 ticking off every criterion.

In the aftermath of that war, Hasan Zeyada, a psychologist with the GCMHP, told the Guardian: "The majority of children suffer many psychological and social consequences. Insecurity and feelings of helplessness and powerlessness are overwhelming. We observed children becoming more anxious – sleep disturbances, nightmares, night terror, regressive behaviour such as clinging to parents, bed wetting, becoming more restless and hyperactive, refusal to sleep alone, all the time wanting to be with their parents, overwhelmed by fears and worries. Some start to be more aggressive."

A study conducted by Unicef, the UN agency for children, following Operation Pillar of Defence in 2012 found that 91% of children reported sleeping disturbances during the conflict; 94% said they slept with their parents; 85% reported appetite changes; 82% felt angry; 97% felt insecure; 38% felt guilty; 47% were biting their nails; 76% reported itching or feeling ill; and 82% were either continuously or usually in fear of imminent death.

This time, says Zeyada, it is likely to be worse. "Any child above six years old has now been exposed to three wars. We are talking about a traumatised generation. They will perceive the world as dangerous, and they will have a lot of frustration and anger. And a desire for revenge." The current war, he says, is "more intensive, more brutal" than the past two. The two main pillars of Palestinian society – family and religion – have been relentlessly targeted. Scores of families have suffered multiple deaths; thousands of homes and dozens of mosques have been destroyed.

Zeyada has observed insecurity among his own four children. "My daughter covers her eyes and ears when the television shows pictures. If I go to fetch bread, my son calls me many times, shouting 'where are you?'"

His family has not escaped the waves of death that have swept over Gaza. Three weeks ago, his mother, three brothers, a sister-in-law and a nephew were killed in an air strike in central Gaza. "There was no warning," he says. Now, in addition to his own children, he is caring for several grief-stricken nephews and nieces.

"No one can guarantee this won't happen again," he says. "If we could guarantee security and safety for children, it would help them to overcome their trauma. But if we cannot protect them from another cycle of violence, it is very difficult … Israel is all the time in the process of creating a new generation of enemies."

Back in Beit Lahiya, Najia – still in pink pyjamas in the middle of the day – sees for the first time the photograph taken of her on the morning of the school shelling. Confusion and anxiety flood her tense little face. Recovery, if it ever comes, is a long way off.