Ok .. huh .. well, just a couple of points. :) don't take this as a flame, I'm from the UK and have a few points of interest you may like to know.
Within England/Wales, criminal responsibility is considered the age of 10 years. This is when a child becomes criminally responsible for their actions and the consequences of their actions. From this age onwards, they can be prosecuted for any criminal offence in a Youth Court. In exceptional circumstances, most notably the case of the murder of James Bulger in Liverpool in 1993, children can be tried as an adult in an adult court.
Basicaly for Harry it would mean he could have killed the Dursleys in front of a witness and gotten away with it. The most that would have happened would have been Harry going to a care home until he was 18. A care home here is nothing more than a hotel for kids with a few rules and adult supervision, they are free to go out and play, go to school, get pocket money, presents on birthdays and x-mas. Heaven for Harry. Or they could go into foster care shrug. Stupid if you ask me, but thats the law here. 7 year old Johnny Rambo could mow down a bus o' nuns while on TV and walked.
Sorry to say this massive, massive blunder kinda turned me off the story a quarter way through chapter one.
My word of advice would be if your going to write a story about a legal system your not familiar with, either ask a native or check Google.
I'd have had the Dursleys die on holiday or work related in Pakistan say, and since he can't speak Urdu or them English they'd just lock him up in an Adult prison, no juve there, until Albus to the rescue! at age 11. But by then the 4 years would have turned him into a vicious little bastard, not evil, just a complete and utter cunt that would kick you in the balls at the first insult or shank you to steal your smokes and be totaly untrustworthy of other males.