First, as said, it is not criminal damage. The mens rea, the mental element, that has to be proved is that the person intended to destroy or damage property or was reckless as to whether it was. The charge fails on both elements. The CPS would take the legally sound, and common sense, view that two schoolchildren fighting have no reason to think, if they did think, that a car bumper is going to be damaged; it is not a reckless act with regard to the property. 'Reckless' is to cover such cases as someone throwing a brick at a shop but damaging something in front of it; they may not have intended the damage ,or any damage, but anybody could see that there was a great risk of it. It was plainly forseeable.
if it's not criminal damage what is it? Nothing. Name me a provable offence. Affray? No: it has to alarm a person of reasonable firmness. Assault? Not if it's a consensual fight; you'd have to prove who the initial attacker was otherwise. It might be conduct which amounts to a breach of the peace at common law, for a bind over, but any justices' clerk would raise an eyebrow at that and both eyebrows if the parties are juveniles!
Face it. It's a non-starter as a civil or a criminal case.