What Jolly Roger said. You're comparing apples and oranges.
If a 4 year old found a gun, played with it, shot someone, then I would have no strong objection to a civil lawsuit naming the four years old as a defendant (not accused, that's criminal), nor would I be particularly concerned with the same 4 years old being found to be old enough to know that what they were doing was wrong. This is because the judge only has to determine which is more likely based on the evidence presented him (including, presumably, actually talking to the girl).
Make it a criminal trial instead, and I would have a much bigger case of raised eyebrow at the girl being found responsible for her actions, would it only be because the burden of proof moves from "most likely of all possible options" to "proven beyond reasonable doubt", which is a lot harder to make, and a lot easier to defend against.
As for sentencing, yeah, what JollyRoger said.
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.Shane. - As far as I can see, in practice, that's pretty likely to be the end result here: even if the girl is found responsible, it's likely that her parents will be responsible for the bill; and of course they'll be the ones who will handle the actual trial. But she has to be named as defendant because the suing party hold that racing bikes in the streets was a wrong action that resulted in negative consequences for them; and that's what SHE did, not what her PARENTS did.
(What her parents can be sued for, conceivably, is failing to educate her about it being wrong to race bikes in streets; and failing to watch her well enough and keep her from racing bikes in streets).