In the US, there is no right to trial by jury in juvenile court. I don't remember the SCOTUS case where that was decided, but IIRC the basis of the decision was that violations committed by juveniles are not "crimes," and therefore do not fall under "the trial of all crimes shall be by jury." While this is arguably a reasonable decision based on the letter of the law, it seems to obviously contradict the intent. Why would the government not want suspects under a certain age to be tried by a jury?
The government has the choice
They can prosecute the child for a crime in the adult system and the defendant then has the right to a jury, or they can refer the matter to the juvenile justice system (JJS) in which case any sentence is administrative and rehabilitative, not criminal and punitive. Some jurisdictions have removed certain classes of crime from the JJS and others allow the prosecutor or the JJS judge to refer the matter to the adult system.
You may argue that this is a distinction without a difference, however, SCOTUS did not agree in McKeiver v. Pennsylvania (1971).
The fifth amendment says "No person shall ... be deprived of life, liberty, or property, without due process of law" and SCOTUS was satisfied that the JJS provided that. They were also satisfied that because the prosecution was not criminal, the sixth amendment's right to a jury trial was not engaged.
The JJS was established around the turn of the 20th century out of a belief that juveniles were more amenable to rehabilitation and that juvenile crime was a product of lack of parental supervision and societal influences whereas adults made a conscious choice to be criminals. In theory, the idea was to create a more sympathetic and less adversarial system to allow orders that were aimed to promote rehabilitation rather than punishment. Its success in that regard has been, at best, mixed.
With the rise in crime in the US (but also worldwide) from the 1970s to 1990s, it became more politically beneficial to be "tough on crime" and more children were diverted from the JJS to the adult system - especially if they were people of colour. Even though crime rates have crashed since the turn of the 21st century, this is still many politicians' go-to response.
Because there are competing public interests. Jury trials are public and produce a permanent public record. There is (at least in contemporary US society) a widely held belief that children are not fully responsible for their actions, in part due to the physical immaturity of their brains. Accordingly there is a desire not to see a child suffer life long effects for an impulsive act, or a situation where a child may not have fully understood the consequences of their actions. Therefore juvenile court records are generally not public and their records sealed.
As @DaleM 's answer pointed out, the government has some discretion in choosing between the juvenile and adult legal system. Particularly for serious crimes, the judge may hold that the crime and the planning of the crime shows evidence of mature reflection and deliberate action, and hold the juvenile responsible as an adult.
The juvenile system is intended to keep the proceedings out of the criminal justice system where people are put at risk of life and liberty.
Since that has succeeded (otherwise the case would not be being heard in the juvenile system) the question is what is the purpose of the JUVENILE court, and that is the best interest of the child and society, the purpose isn’t justice or revenge or making the victim whole. It’s not so much rehabilitation as the proper treatment of the child in order to prepare them for the transition to adulthood weighed against or in conjunction with keeping society safe. Having two sides makes sense as the best interest of society may not be the best interest of the child and vice-a-versus. More, just needlessly complicates things.