It’s less than two months since the federal election, but the Greens already have a concrete achievement to show their supporters: not just talk, as with the new climate change committee or the debate on the Afghan war (although talk can be important too), but a real policy shift, with the move to take families with children out of mandatory immigration detention.

The government, not surprisingly, denies that the Greens had anything to do with it, and it’s probably true that the new policy was driven mainly by logistical constraints. But given how comfortable the government had seemed in pandering to xenophobia before the election, it’s hard to believe it would be highlighting this particular change just now if it were not for the changed parliamentary dynamics.

Moreover, the change itself is pretty modest. Even the Howard government disclaimed any desire to keep children in detention; the fact that this can now be presented by the Greens as a breakthrough is a measure of how far the debate has deteriorated just in the past few years. Clearly their power in the new quasi-coalition is not quite what the scaremongers would have us believe.

Nor is there any obvious logic to why children should be treated differently. Of course, child refugees are innocent parties — but so are the adults. Detention is not supposed to have anything to with guilt; the point of the policy, if there is one, is deterrence, and locking up children is probably a more effective deterrent than locking up adults.

It can’t be said too often that these people are not criminals: they are not “illegals”, or “economic migrants”, or security threats, or any other such nonsense. The vast majority, as everyone acknowledges, are genuine refugees, with a legal right to apply for asylum in Australia.

No other democracy finds it necessary to keep such people in mandatory detention. And this, surely, is the debate we should be having: not why are we locking up children, but why are we locking up anyone, beyond the minimum necessary to conduct health and security checks.

And this is the problem the Greens face: their influence in government is not a free gift. It comes at a price, the price of a reduced ability to put forward more radical solutions.

The Greens are the ones we would normally rely on to argue against the whole system of mandatory detention, the issue that drove the big jump in their vote in 2001. No other party has clean hands; it was the Howard government that ratcheted it up to absurd heights, but the policy was first introduced by the Keating government, and another Labor government now seems happy to treat it as part of the furniture.

But this time round, the Greens’ voice is muted. It’s hard to simultaneously convey pride in a policy achievement and dissatisfaction about how far there still is to go — and that’s especially difficult for a minor party whose media exposure is severely limited in the first place.

The point is not that Greens’ supporters will see their leadership as selling out — that may at some point become a problem, but as well-educated voters, they should be able understand something of the compromises that power requires. Certainly if the Greens want to be a major player, it’s something they will have to deal with.

The real problem is that if the Greens move further into the mainstream, we will miss their advocacy of unpopular or unorthodox solutions. Political debate may be the poorer for the absence of a strong voice for radical change.