I think you misunderstand the juvenile justice system. Short sentences in juvenile hall or community service are handed down only for very minor crimes. For something like this - attempted murder, arson - a juvenile offender could be sentenced to several years in a youth correctional facility (read: junior prison). In most states, for the most serious crimes juvenile offenders can be held in such facilities until they're 21 years old.
I'm guessing though that that's not deterrent enough. So consider the way kids think. Thinking ahead, considering the consequences, things like that are part of adult thinking. One of the things that makes children "childish" is that they don't do this so well. They can sometimes be deterred by an immediate and well-understood threat ("Dad will ground me for life if I scratch his car"). But often even that isn't enough. The far-off possibility of being sentenced as an adult surely isn't likely to deter the sort of child who would set another child on fire.
So I don't know that the deterrence angle is such a good one.
