Sorry for updating a thread that's gone quiet but is there any chance you could let me know how this panned out?
For what it's worth, I would have done exactly what you were proposing to do. I'd view it as a racing incident with no liability for your son and I'd have made that point politely but firmly. I would also have taken the long view that it's better to keep things running smoothly and offer to make a small contribution to the repair. If I was in a particulalry grumpy mood, I might also have made a veiled comment about children training on carbon frames but there you go ;-)
The reason I ask is that a my much younger brother and his friends are in a similar situation. They were doing through and off training when a less experienced thirteen year old pulled off and tapped the brakes a mite hard resulting in a pile up. My brother's frame took a beating but, as it's basically made of scaffolding poles (much to his continued annoyance), it's completely fine. The parents of one of the other children are asking for money from the parents of: (i) the child in front who tapped the brakes (fair enough on one level but it's still a racing incident in my mind); AND (ii) the child behind who went into the back of their child (a bit off in my view).