Seems like a perfectly reasonable sentence to me, given their ages.
A few years in secure care away from the influences that lead to them acting like that, at such a critical point in their lives has every chance of making them properly productive members of society. any longer would ruin those chances, and cost society in the long run.
So you are saying that being in prison takes them away from bad influences?? As in prison builds character sort of way?
They killed a person who was not to blame for anything. Where is the justice for the victims family? Or is it that suddenly custodial sentences look only at rehabilitation without retribution for the victim? A sentence needs to be retribution-punishment-rehabilitation, all three and in that order. As the perpetrator you have an obligation to satisfy the victim or victim's family FIRST before you get your chance at freedom.