Could be they were on the same belt and skill level and thus were matched to spar each other, regardless of size/age difference.
That's a possibility.
also means I'd be matched up with a fetus if I participated.
Yeah, gosh, I certainly hope that the karate class would match everyone up with someone who is a fair (and safe) match.
I don't know much about Carl, but he seemed like an intelligent enough guy that he wouldn't have kept his kids in a class where the instructor was insane and matching near-adults up with kids unfairly.
You'd be surprised. What happened there was nothing. I know of real horror stories.