You can disagree with it, but there's honestly nothing about this story that's all that shocking at all. Juvenile justice tends to err to either extreme - kids get tried as adults and people get pissy about it because they feel like kids should be rehabilitated rather than punitively punished, or they get minor sentencing due to their age and people get pissy because they feel like they didn't pay for what they did.
You really start to get into a grey area over 16 or so, but when you're dealing with people under 16 there's always going to be different standards.
The prosecutors wouldn't even try them as adults. Plenty of 15 year olds have been tried as adult and gotten prison time, and for crimes less heinous than this. No one expected them to get life in jail or anything, but this is outrageous.