The sentences people receive are highly dependent upon the judge and the county. All I can say is that these are serious charges and you should have a skilled attorney to assist you. Having an attorney will most likely reduce the sentence you will receive.
Every case is very fact dependent and so it is very hard to tell whether one would be able to obtain a minor conviction or not. That being said the fact this is a juvenile case and not an adult case means that the focus should be more on rehabilitation than punishment.
The DA is going to be looking at the damage done and the cost to clean up the graffiti. Restitution is going to be an issue and if there is anyway to have paid off at least part of the costs that should have a positive impact on the case.
Whether the juvenile will be obtaining a private attorney or public defender you can be better prepared by putting together background information on the juvenile. Is the juvenile doing well in school? If yes, bring school transcripts to the attorney. Is the juvenile involved in any community organizations? If yes, bring letters in support from these organizations.