Adult members of two families have been ordered by a judge to be immediately fitted with electronic monitoring tags because of fears they could take children to areas controlled by Islamic State.
Sir James Munby, president of the Family Division of the High Court, ruled the tagging measure - believed to be the first of its kind - is necessary to end the children's separation from their parents whilst ensuring their safety is protected.
Local authorities who placed the children in foster care opposed them being returned to their families pending full fact-finding hearings of the cases in the family courts later this year.
But the country's most senior family judge ruled that the the risk of them being taken from the UK was "very small indeed" and counter-balanced by the children's need to be returned to parental care.
Sir James announced he was intending to make the tagging orders as part of a package of safety measures in a judgment he gave last week.
But he postponed making the orders until today to consider concerns about the monitoring raised by the Ministry of Justice (MoJ), which described the situation as "unprecedented".
The judge said the case arose after two family groups were detained at airports - one in the UK and one in Turkey - earlier this year when security officials became concerned they were heading to Islamic State areas in Syria.
He said social workers had taken the cases to family court in a bid to protect the children.
Youngsters were placed in foster care pending decisions about their long-term futures, due to be made at hearings in October and December.
Sir James said he had to decide whether youngsters could be returned to parents pending future hearings and final decisions.
He said he had concluded that they could - despite social workers raising fears that children would be taken to IS areas and their lives put at risk - provided "comprehensive" protection packages were in place.
He said parents had agreed to submit to a range of restrictions - including electronic tagging, living at specified addresses and regularly reporting to police - if children could return home.
"I accept that there is some degree of risk of successful flight," said Sir James.
"But, taking a realistic view, though not forgetting that we are here in the realm of unknown unknowns, my considered assessment is that the degree of that risk is very small indeed, so small that it is counter-balanced by the children's welfare needs to be returned to parental care.
"I should add, to make plain, that in relation to their welfare (leaving flight risk on one side), the benefits all of these children will derive from being returned to their parents clearly, in my judgment, outweigh any and all of such contrary welfare arguments."
The judge ruled that radio frequency (RF) tagging - plus an additional curfew period - should be used until GPS tagging, which offers a greater measure of security, can be put in place in a fortnight.
The local authorities involved had decided the risks of returning the children were too great and they would not be able effectively to manage them.
The children are now wards of court and it is the court that has responsibility for making decisions on their behalf.