The number of children with learning disabilities and autism locked up in mental health hospitals has more than doubled in four years, despite government promises to halve detentions.
A new report by the Children's Commissioner for England found that in February there were 250 youngsters detained, compared to 110 in March 2015.
In 2015 the government promised to move up to 50 per cent of children and adults into community-based support systems by March this year, but NHS England recently announced it was extending the deadline by five years.
The new report found too many children were admitted unnecessarily to mental health institutions, often far from home, and were regularly restrained of sedated by staff. In December there were 750 reports of children being drugged or bound to keep them under control.
Anne Longfield, the Children's Commissioner, said the inquiry had found "shocking evidence of poor and restrictive practices"
"I will never forget the stories I heard from mums and dads at a meeting I arranged for parents with children in these units and their tears of frustration and anger," she said.
"Some of them have a child who has been locked away in a series of rooms for months. Others have to listen as they told by institutions that their child has had to be restrained or forcibly injected with sedatives. They feel powerless and, frankly, at their wits end as to what to do.
"The onus is now one ministers, the NHS, the Care Quality Commission, Ofsted and local authorities to make sure that these most vulnerable of children are not locked out of sight for years on end simply because the system is not designed to meet their needs."
The report found that the average child detained in a mental health institution had been there was six months, while around one in seven had been there more than a year, and some for more than two years.
Nearly one in four had been placed in a hospital for six months that was at least 31 miles (50km) from their home, and one in 10 had been housed 63 miles (100km) from their families.
Parents complained that their children had been neglected with one family saying their son had not been washed for six months while in hospital.
Some children also had their bedding taken away during the daytime to prevent suicide attempts.
Many children were detained under the Mental Health Act because they were a danger to themselves or others, but a lack of places in the community, or staff to support them at home, mean many are not allowed out, even when they are no longer a threat.
One mother, who was not named, said her daughter had been kept in a mental hospital for two years. "She's got to stay there until they find her a place. She doesn't need to be in hospital any more but she's not allowed to come home.
"We would like her to be somewhere close to home that we can see her whenever we want and that's she'll be safe and no one can take advantage of her."
The Children's Commissioner has called on Ministers to launch a national strategy to ensure children do not have to go into hospitals in the first place. She also called for NHS England to monitor all detained children and bring in incentives to reduce the numbers.
Mrs Longfield added: "Hospital admission must be in the child's best interest and as part of a managed process, with clear timescales and a focus on keeping the length of stay as short as possible.
"This is clearly not happening at the moment and instead we have a system that is letting these children down."
A Government spokesperson said: “Autistic children and children with learning disabilities must receive high quality, safe and compassionate care.
"We are determined to reduce the number of autistic people or people with learning disabilities in mental health hospitals - significant investment in community support has already led to a 22 per cent reduction since March 2015. "
A separate investigation by BBC Newsnight found record numbers of teenagers were being looked after in unregulated homes.
New figures show the number of unregistered or unregulated homes for children has risen by 70 per cent in the past ten years, with more than 5,000 over-15s living in them, compared with 2,900 in 2009.
Children and Families Minister Nadhim Zahawi said: "Children in care or those leaving care, including older children aged 16 and 17 deserve stability in their lives including good accommodation."