IMO - The problem is the steep and slippery sides of these very deep ponds, which are popular in Isaan. I suspect that the kids either go for a swim or fall in, and just can't climb out. Eventually they are so exhausted that they go under. It seems fairly common that one child gets into difficulty and others drown whilst trying to help.
Looking at Owl's photos, his ponds seem to be shallow at the edges, which would make it easier for someone to climb out. Maybe there's some reason why the deep/steep-sided ponds are preferred by some. Maybe someone can explain.
I have mentioned previously that my wife had one of these ponds dug for free on her small farm a few years back. The soil (clay) was taken away for roadworks. Last month she had the pond backfilled again with clay as it never produced any fish worth the trouble.
When I read about these tragic drownings, I feel relieved that we will not have any kids drowning on her farm.