The mental and physical health of children held in asylum seeker centres is being put at risk, a charity says.
Up to 2,000 children a year are held, the charity says
Save the Children said detention before deportation can cause problems like depression and eating disorders, and harm educational chances.
It has urged ministers to rethink the policy of using one of four removal centres in the UK.
The Home Office defended detention of families - but said it would consider the findings "extremely carefully".
The report estimates 2,000 children are detained every year in the centres, which hold failed asylum applicants, or others breaching immigration rules, prior to removal from the UK.
REMOVAL CENTRES WITH FAMILY UNITS
Dungavel: 56 beds
Yarl's Wood: 232
Tinsley House, Gatwick: 32
Ministers have expanded the use of detention as part of asylum enforcement measures.
Holding of children in these circumstances remains controversial after a string of campaigns and the prisons watchdog Anne Owers questioning the policy.
Opposition to child detention remains strongest in Scotland because of the family unit at Dungavel Removal Centre, Lanarkshire.
Based on interviews with families, people working in the system (including Home Office staff) and visits to centres, the researchers found some children had been held for more than a month and a smaller number for up to nine months.
In some cases, families were being held despite waiting for an outcome on their asylum applications.
The report said there were "significant gaps" in available information, meaning it was difficult to establish exact numbers being held, although it estimated the numbers to be 2,000 children a year.
At present the Home Office publishes a quarterly snap-shot of those held on a particular day, but no complete annual data.
Mental health concerns
Parents reported children refusing to eat properly, lacking sleep, developing coughs seemingly associated with detention and an unwillingness to learn.
The researchers also raised concerns about the detention of asylum seeking minors who had arrived without parents.
They said there were no figures on the numbers held as adults later found to be younger than 18-years-old.
Dungavel: Centre holds families in Scotland
One study by Cambridgeshire social services found almost half of age dispute cases to be under-18s.
Heaven Crawley, author of the report, urged ministers to stop detaining children, saying other countries had found workable alternatives.
"The Government argues it is necessary to detain children in order to maintain the integrity of UK immigration control," said Dr Crawley.
"Our research shows very clearly that many children are detained for the purpose of administrative convenience or at a stage when all of the family's circumstances have not been fully considered."
Dr Crawley said ministers should consider the US system under which families are closely monitored and supervised in the community up to the time of removal.
Responding to the report, a spokesman for the Home Office defended the detention as necessary to ensure the immigration system effectively dealt with those whose claims were unfounded.
"We welcome the report, which the Home Office contributed to, as it makes a valuable contribution to an emotive debate," said the spokesman.
"Detention of families is kept to a minimum and is subject to frequent and rigorous review. Very few families are detained for more than just a few days.
"This is a detailed report and we will need to consider the contents and recommendations extremely carefully before responding in full, which we will do in due course."