my two cents:
i don't understand how if the kid was going full speed, and hit you square in the rear derailluer, you didn't fall down too. the kid fell down, and obviously struck you with enough force to break your frame...that should have knocked you down, unless you slowed down enough that you could catch yourself, in which case, you made the wrong decision...anyway...
i think the kid should have been paying attention, and the parents should have to pay some of the cost, not all. if you saw the kid before you got to where he hit you, you could have avoided it, especially since he hit the rear end of your bike. i'm not saying you're in the wrong or not attentive, but it probably could have been avoided if you had anticipated a collision. it also would have been avoided if the little snot had been paying attention, but hey, the kid is 12, and kids screw up sometimes, it's not like he broke your leg or something...
if it was me, i'd be pissed, but i'd take it to a shop BEFORE calling the guy to make sure any repairs would be worth charging the family for. also so you can say to the guy positively what needs to happen. a broken derailluer hanger is not worth the hassle, and not really worth going to small claims court. if you have to get a new frame, then i'd say make them pay at least 3/4 of it for being neglegent parents, but don't blame the kid. if the derailuer isn't replacable, you can always get one of those stamped steel "feet" that attach via the QR (they used them a lot on cheap road bikes in the 70s and 80s) that will hold the rear derailluer just fine it might not be the coolest thing, but it'll work...of course you'll probably have to get a new dreailluer too, which i would make them pay for some of, and really drive it home that the kid shouldn't be out in the street and there should have been someone watching him. make them feel bad in their hearts, not in their wallets.
oh yeah, the comment "tell them next time you will be a car" wasn't funny to me either. there are some things you just don't joke about.