Should children be treated as adults?

Nowadays there is a lot of commotion around a murder case of a thirteen year old girl. The case is still not clear, but three people are accused of  having commited the fact: one adult, a 16-year-old and a 12-year-old.
Under our law system, the 16-year-old could face a murder charge before an adult court - if the youth council agrees on it.
But the 12-year-old can't be punished. Most that can happen to him is being placed in a foster family.
Makes me wonder why this is so.
I don't make a judgement about the two youngsters concerned in this case, but I have known for fact that young people can be just as malicious as grown-ups.
In my long teacher career I've met some boys and girls, of whom I knew for sure they would become criminals. I don't know how, but I can judge a person's character from when I first lay eyes on them. These youngsters later on proved me right. One became a rapist, the other a murderer, another one did hold-ups where people were left for dead.
In my view - and this is solely mine - children who commit crimes like murder or rape should be treated accordingly. They should be punished, just as adults should also be punished harsher than they are now.
Did you know that in Belgium 'life-long imprisonment' only means 30 years? And that the criminal can be free after 20 year, when he or she behaves well in prison?
No wonder people take to protesting. Lots of them don't agree that Michelle Martin (ex-wife of Dutroux) would be set free and can retire to a convent.
What's your view on this?