Quite so, em. Victims of violent crime are entitled to see their attackers dealt with robustly and issuing a caution does not meet that definition in my view.
This matter is complicated slightly by the fact that the alleged offenders are under 18. However, here’s an extract from the Code for Crown Prosecutors that deals with young offenders:
"Prosecutors should not avoid a decision to prosecute simply because of the suspect's age. The seriousness of the offence or the youth's past behaviour is very important."
"Cases involving youths are usually only referred to the prosecution service for prosecution if the youth has already received a reprimand and final warning, unless the offence is so serious that neither is appropriate or the child or young person does not admit committing the offence."
It is my view that this offence was “so serious” that a reprimand or warning was not appropriate. It is also my suspicion (though of course I do not know) that this matter was not properly put before the CPS.
By the way, there was a slight error in my earlier answer. “Conditional Cautions” cannot be issued by the police but must be dealt with by the CPS.