Tity Nzamba Bolele said her children were "not okay" in the removal centre
The UK government's treatment of the children of asylum seekers in detention is "abusive" and "dehumanising", it has been claimed.
The chief executive of the Welsh Refugee Council said the government was in breach of the United Nations Convention on the Rights of the Child.
Mike Lewis said that children are often split from their parents.
But the UK Border Agency denied the claims and said it did not recognise the description of how it worked.
Referring to early morning removals of families from their homes, Mr Lewis said: "For us the trauma of that experience is pretty... I would use the word abusive actually, because I don't think its done from a child-centred way and shouldn't be happening in the UK."
Mike Lewis said he believed the UN Convention was being broken
The Welsh Refugee Council has said children are sometimes separated from their parents in the process of a journey of several hours to a detention centres in England.
Mr Lewis said: "They don't know where mummy and daddy are sometimes. And these are quite young children.
"We're not talking about 15 year olds. We're talking about three and four year olds, babies even.
"We've got stories of children who haven't been fed all day in this process.
"They then go into these places where they could be there for months while their claims are sorted out. I don't think you could make it any more dehumanising really".
The United Nations Convention on the Rights of the Child stipulates that detention shall only be used as a last resort for the shortest appropriate period of time.
See how one family coped during their time in a removal centre
But the UK Border Agency's Regional Director for Wales and the South West said he did not recognise Mr Lewis's description of the way his officers work.
Phillip Smith said detention was always a last resort after people had been given opportunities to return to their country of origin voluntarily and with assistance.
He denied that people were deprived of opportunities to eat, drink and use the toilet, and said families were only very rarely split.
When they were separated, it was to ensure the safety of the family and immigration officers when an adult resisted removal violently.
Mr Smith said: "I think it can never be pleasant to be taken from your home and required to be somewhere else.
Children have not committed any crime and yet they're being treated as if they were criminals and being locked up.
Jenny Willott MP
"We do all that we can to make that as bearable and as untraumatic an experience as we can.
"The family wings in our removal centres are furnished appropriately for the needs of children.
"There's education available to them on site. There are recreational and social facilities."
Tity Nzamba Bolele and her children fled to Cardiff from the Democratic Republic of Congo to seek asylum.
She fears they will be killed if they return to Congo.
Her family's application for asylum was refused.
Daniel and David Bolele were in a removal centre with their mother
Seven immigration officers removed them from their home shortly before 0700 GMT on 26 January.
Ms Bolele and her children, aged three, five and nine, were taken to Yarl's Wood Detention Centre, Bedfordshire, and detained for 18 days.
Ms Bolele said: "It was a very painful experience in the prison.
"We found ourselves in a place where, wherever you come out, they open the door, they lock after you, they open the door, they lock after you.
"And the children were not okay. They were asking me, 'We want to go to school'.
"There were no schools. They were not eating good food. They couldn't really play. They are being really stressed."
Cardiff Central MP Jenny Willott says she was ready to take the Welsh Refugee Council's claims up with the government.
She said: "I think children are treated very very harshly in the asylum system. I don't think we should be detaining children at all.
"Children have not committed any crime and yet they're being treated as if they were criminals and being locked up.
"And I think it puts them under immense stress. It can be an extremely upsetting and distressing experience for them and I don't think we should be treating children like that."
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