The federal government's counter-terrorism legislation amendment bill drops the age when control orders can be imposed from 16 to 14.
The legislation has passed the Senate with the support of Labor.
Liberal Democrat senator David Leyonhjelm stood in opposition.
"Because it significantly, permanently and unjustifiably expands the power of the Commonwealth government to hold people without charge."
The Senate agreed to amendments that require a court to appoint a lawyer to act for minors under 18 if they do not already have one in proceedings relating to a control order.
Attorney-General George Brandis says the change will further enhance protections for young people.
"At the moment, in relation to 16- and 17-year-old people who are subject to the existing control order regime, there is no special provision to guarantee the appointment of a lawyer. Now, as a result of these amendments, for people between the age range of 14 and 17 years, there will be."
In backing the amendment, Labor senator Kim Carr says the party has sought to strike a balance between keeping Australians safe and preserving civil liberties.
"Where appropriate, the Labor Party will seek improvements to legislation in line with our values. And we do so, however, with the foremost priority being to ensure that we are able to protect the safety of our community."
Greens senator Nick McKim has expressed concern about children as young as 14 being made subject to control orders.
But he says he is happy to back the provision of lawyers to minors under 18.
"Because we do think that it enhances the safeguards for children, although this improvement is certainly by no means enough to change our view that this is bad legislation."
One Nation senator Pauline Hanson says legal assistance should be subject to means-testing of the parents.
"Someone has to start taking responsibility. And we do in many other areas, the parents are responsible for their children. Why not in this area?"
Mr Brandis says only six control orders have been issued in 12 years, making it a highly unusual procedure.
He says means testing is not practical.
"We are dealing with a very unusual procedure, and often a procedure that takes place in very urgent circumstances, in which the practicalities of establishing whether or not the means of the parents of a person who is the subject of such an application would make it, I think, difficult, if not impossible, for the system to be workable."
Crossbencher Nick Xenophon points out that, in some cases, the minors may have no relationship with their parents.
"There may be instances where they have had their minds poisoned, have been radicalised, have got absolutely nothing to do with their parents, that they are in the clutches of extremists."
