There have been a few stories like this emerging from Japan, particularly stories involving Sumo. I've even read a scanned document detailing deaths in Japanese Aikido classes caused by training. Whether or not this is a more serious problem in Japan compared to other countries, I don't know. Perhaps stories from Japan get reported more, who knows?
Too many martial arts instructors don't get that kids are kids. You can't train them like adults. I see kids in traditional MA classes being forced to hold stances and walk in line for 90 minutes while being shouted at by some wannabe drill-sergant in white pyjamas. I see clips of 8 year olds in a cage doing MMA training, telling the camera they "love" to train while their body langauge and their "suportive" parents body language tells a differnt story.
What I find bemusing though is that many people in the West lament that martial arts have been "watered down" and that health and safety concerns mean martial arts aren't "deadly" anymore. It's BS in my opinion. I'd rather a kid learn a "watered down" martial art and stay safe within the class than get badly injured or worse doing "da real thing".
The Judo sensei in the story is an extreme example of what is all too common in MA classes all over the world: Instructors not treating children in their care properly. It's an utterly senseless loss and I sympathize with his family.
"Let your food be your medicine, and your medicine be your food"