I used to work with someone who campaigned for the ending of immigration detention of children in the UK.
The UK's currently down to 71 children in immigration detention in 2016, although it's been much higher in the past. (www.migrationobservatory.ox.ac.uk/resources/briefings/immigration-detention-in-the-uk/)
When families with children are detained, there is no policy to separate them. In fact, what there's been arguing over is the whether the accommodation for families-with-their-children is suitable.
Eg. Government accused of scrapping pledge to end child detention in prison-style immigration removal centres
www.independent.co.uk/news/uk/politics/child-detention-immigration-centres-scrapped-broken-promise-tinsley-house-cedars-barnados-home-a7149981.html
Cedars was the UK’s first pre-departure accommodation specially designed for families being removed from the UK. Support there is provided by the children’s charity Barnardos and it features apartment-style accommodation for families and a welcoming appearance, resembling a suburban house.
The families and children living there will now be moved to a self-contained unit at Tinsley House removal centre – a secure detention centre surrounded bya chain-link fence, run by G4S that resembles a prison.
Lisa Nandy, a Labour MP, said the policy announcement effectively amounted to the scrapping of the Government’s pledge to end child detention. “On the last day of Parliament Ministers quietly abandoned the promise to end the immigration detention of children,” she said. “Totally indefensible.”
I'm not happy with the moving of families back to Tinsley House, but it's a million miles from the Trump policy of intentionally separating all children from their families and throwing them into warehouses unaccompanied. That's before we even get to the bit where the Trump admin doesn't have a mechanism for reuniting the children with their parents, and admits the babies it took may never go back to their parents.