I'm referring to any version of seoi nage but maybe seoi otoshi even more. It seems to me the danger of landing on your head and breaking your neck is greater than with the kubinage, especially as those kids were doing it. And while there is danger of straining the neck or worse if uke isn't off balance, I think the greater danger is in being thrown right on your head.
I trained in judo waza for many years before I got shodan in Japan, training under Minoru Mochizuki.
Best to you.
Any throw that lands you on your head is dangerous.As it happens years ago I took severe throw which landed me on top of my head.the pain was excrutiating .Years later a xray technician indicated I may have fractured/broken my neck a long time ago.
At the moment I am suffering from some cervical pain ,hence my concern when I see waza like Kubi Nage being done .
Hope you are well, cheers, Joe,