That’s a loaded question bound to get a few responses. It all depends on what we mean by ‘work’, and what we think jail is supposed to achieve.
One stated aim of jail — and any sentencing option — is to reduce offending. Surprisingly, there’s not a lot of research to see if jail achieves this or not.
The Australian Institute of Criminology published a paper last week, The specific deterrent effect of custodial penalties on juvenile reoffending. (One of the authors is Don Weatherburn, Director of the NSW Bureau of Crime Statistics and Research (BOCSAR), and renowned authority on sentencing issues.)
The tentative conclusion is that sending children to jail doesn’t produce any noticeable affect on re-offending rates.
The authors were careful not to say anything more than that. No doubt they’re well aware that jail, and sentencing generally, is used for other reasons too.
In Victoria, the Children, Youth and Families Act 2005 s 362 provides that the Children’s Court may sentence for prescribed purposes — including protecting the community — but not punishment or denunciation.
Cases such as DPP v Ty (No 3) (2007) 18 VR 241 (a 14½-year-old child convicted of murdering an 18-year-old boy by penetrating his skull with the steel tip of an umbrella), and DPP v SJK & GAS  VSCA 131 (an 18 and 16-year-old guilty of manslaughter of a 73-year-old woman in her bedroom) are subject to different sentencing considerations because they are dealt with in the Supreme Court, and hence subject to the sentencing considerations in the Sentencing Act 1991 s 5(1).