Ubiquity If your comment about ranking was aimed at me (I'm not sure?), I wasn't doing that at all. I was just saying the cases are not at all similar.
In Norway, two six year old boys are playing with a five-year-old girl, something happens (most people seem to believe a tantrum), and they beat her up. They seemingly had little intent to kill her, they thought she was sleeping. They were looked after in a way anyone would expect them to be - lots of help from school, the authorities, the parents.
Our nearest comparable case in the Bulger case. They were 10, though, and they planned to abduct and kill a child. They kidnapped him in a shopping centre, and tortured him. That's not comparable, at all.
Then in this case, there's two 12 year olds - double the age. The elder one seemed to genuinely believe that beatings wouldn't kill, but the younger one almost certainly did understand. They chose someone who was vulnerable but whom they knew, who had helped them to get alcohol and given them somewhere to hang out away from their families. They beat her horrendously, and then left for a break, and then came back and carried on. And through the entire attack, they documented it on Facebook and Snapchat.
If you compare the killers - two six year olds to two ten year olds, or two twelve years olds, there's a big difference.
If you compare the crimes, the difference is huge. Norway had a case of an almost accidental murder, they didn't name the children and helped rehabilitate them. The UK might have done the same.
To be honest, it's only the big, graphic and surprising cases that tend to make the news. There may well be cases where six-year-old children in the UK have killed someone, and they weren't subject to the legal system...
And that leads us to the age of criminal responsibility, which is roughly 10 in the UK, but between 7 and 13, can be argued either way - if you can show that you didn't understand the consequences, that's a defence. Likewise, if the prosecution can show you did.
Context is everything, even when victims are "ranked" the same. At the end of the day, you're always going to have at least one person dead, at least one person who killed them, and the how/why/when is the context that says whether the sentence is better or worse. If you strip the context away, it'd probably have to be "kill someone = life imprisonment", which would be unfair in some cases, and brilliant in others.