Legally speaking, I believe the age of criminal responsibility is 10 - below that age, children are judged to lack the capacity to form the mens rea (mental element) which many offences call for. From 10-14 they can be charged with more serious criminal offences but not, I think, all offences. The rationale is various, but in short it comes down to the fact that modern criminology views criminal activity by children as being a symptom of problems in development which they lack the capacity to influence - in the same way we say that kids shouldn't have sex under 16 as they lack the ability to understand it in its broader context or all its implications.
The problem with ages (like the age of sexual consent) is that it's somewhat arbitrary - clearly some kids have capacity at an earlier age than others. I guess the courts try to take this into account as best they can on a case-by-case basis, subject to the legislation.
In this case, I find it pretty hard to see that the children involved couldn't form the mens rea required to be charged with attempted murder - the mens rea being the intent to kill or seriously harm. I would imagine that the vast majority of kids of this age would understand that repeatedly cutting someone with a knife, repeatedly hitting them with a brick and then throwing them down a steep slope into water would cause serious injury.
The question then is, if they're found guilty (as they probably should be on the facts out there), what the appropriate punishment/ treatment should be. You would hope that in determinining this the courts would take into account all the evidence (of which we have none at the moment) - all I would say, is given the facts we do (i.e. in care, history of violence and nature of the offence) it would be very surprising if they had upbringings which were entirely normal.