1.
I feel like the strongest 'for' argument is to give justice to the victim's family (if there is a victim - I'm talking murder).
However, I definitely feel that the child's mental capacity does need to be taken into account. Not only this, but I worry that adult sentences may be counterproductive for children. Surely they'll be made to feel like outsiders from a young age. Perhaps this is enough to prevent the child from proper social integration; maybe it will cause them to 'live up' to how they perceive by themselves and by other people - as a criminal.
I've actually written my first blog post on this, but am not going to link it here because I'm genuinely interested in this discussion. If anyone does want to read it though I've put it in the 'about me' section of my profile.