In a report, the Childhood Trust says disadvantage is leaving children extremely vulnerable.

As well as anxiety about their loved ones' health, many children are facing social isolation and hunger.

Lack of internet access is also setting disadvantaged children back.

With many classrooms still closed for lockdown, children unable to access the internet at home have been effectively shut out of online lessons. Teachers warn this will lead to entrenched inequalities between them and classmates from more affluent families.

Children in this position are also unable to access online therapy or other healthcare appointments they need.

A lack of contact with teachers and GPs, who are trained to spot the signs of abuse and neglect, is also leaving kids who are experiencing abuse at home hidden and in danger.

'Mum's going to die, she's not coming back'

Laurence Guinness, chief executive of the Childhood Trust, told BBC News many children it had spoken to were experiencing "vivid nightmares" about the coronavirus and death - a possible side-effect of post-traumatic stress disorder (PTSD).

These children had been particularly affected by the global death tolls, he said, which had made them worried their parents and friends would die of coronavirus.

"The rising death tolls being reported every day - these kids have seen all of that and internalised it," he said.

The report also quotes Dr Maria Loades, a clinical psychologist from the University of Bath, saying lockdown measures are "likely to increase the risk of depression and probable anxiety, as well as possible post-traumatic stress".

Galiema Amien-Cloete, a primary school headteacher in London, told the BBC she'd also seen parents' anxieties around the coronavirus "transferred to their children".

For children, she added, the loss of routine, contact with friends and regular education is often experienced "like a bereavement".

Children in low-income households are also likely to develop anxiety when one or both parents is a key worker - hundreds of thousands of whom earn below the Living Wage Foundation's recommended living wage, according to BBC analysis. The anxiety is not only because of food shortages and poverty, but also because their parents are in high-risk professions.

One primary-school-age girl from London, quoted in the report, said she feels "anxious because my mum works for the NHS, and I don't know if she's going to catch it or not". Another young girl said every time her mother left the house to go to work she thought, "mum's going to die, she's not coming back".

The trust, which works with about 200 charities, also spoke to children with existing histories of mental health problems, to find out whether they were able to access the support they need. Of the 2,000 children with mental health conditions it spoke to, 83% said the coronavirus outbreak had made their mental health worse.

Under lockdown, community centres and support groups have adapted by moving their services online. However, these are not accessible to children without internet access - for example, those who are homeless and living in temporary accommodation, or in overcrowded housing without broadband.

These inequalities are interconnected, too. Children from black and other ethnic minority backgrounds, are more likely to live in overcrowded housing, making access to mental health help harder. At the same time, their parents are more likely to become seriously ill and to die of the coronavirus, making trauma in the kids more likely.

Mr Guinness told the BBC that children with special educational needs are also particularly hard-hit. For those with autism spectrum disorder (ASD), for example, the loss of extra tuition and their set routines has been "catastrophic". Some parents said their children's development had already slipped back by as much as a year.

'There is no "when this is over"'

Victims of child abuse and child sexual exploitation are also particularly at risk under lockdown, the report states. Former Home Secretary Sajid Javid warned of a "surge" in cases of child abuse earlier this month.

The Childhood Trust also points to a 21% rise in alcohol sales during the lockdown period, and quotes a statistic saying there are 2.6 million children living with a parent drinking hazardously, and 705,000 living with a dependent drinker.

"Children and young people caring for family members with substance abuse and/or alcohol problems may find their physical and mental health, relationships, and educational outcomes significantly more impacted than prior to the Covid-19 restrictions," the report says. This is particularly because of a lack of contact with teachers and health professionals who are trained to spot the signs of abuse.

Headteacher Ms Amien-Cloete told the BBC she believes these issues will continue to affect this entire generation of children well after the coronavirus crisis has passed.

"People keep saying 'when the coronavirus is over'," she said. "But there is no 'when this is over'.

"I think we need to be mindful that this won't be over for a long time, because we will have to cope with the impact of this on children. It's like grief - they say you never get over someone's passing, you only learn how to live with it. We shouldn't think it's all going to go back to normal when there are no more cases."