In response to
this, I wrote:
I believe that "fair" is nothing more than a game-theoretic equilibrium (see Binmore's "Game Theory and the Social Contract" for more on this view). It is impossible to return the wall the original state, and I'm not sure I'd want to live in a world in which the parents of these teenagers were forced to be your slaves because they couldn't afford to restore the wall. It seems that the right thing would for an independent party to assess the damage, even if this ignores the subjective emotional damage to the victim.
When you drive a car around, you have to reckon with the chance (be it 1 in 1 million or whatever) that you may destroy property (or even kill someone).
If, by bad luck, you happen to destroy a Ferrari, should you really be liable for the full damages? I believe that the owner of such an expensive car should normally be responsible for most of the damage in case of an accident, otherwise the car just becomes a potential liability for everyone else, and becomes thus undesirable on public roads.
An accident should be a bad thing for everyone involved: the victim should still be worse off, even after compensation is paid to him/her. This seems like a good game-theoretic equilibrium. It is, afterall, impossible to determine fault completely. If you broke even every time a teenager drove into your wall, then you might put some magical magnetic thing on your wall that attracted teenage drivers just for your entertainment (or, more plausibly, you could just neglect to take preventive measures). Anyway, this doesn't seem to apply to your case, but I'm making a general point.