I've played with the code a bit more, and it's failing for me as well. But not always. How often it fails, depends on the number of spawned children. Trying 1, 2, 3, 4, and 5 children, each 10 times, I get success rates of 100%, 90%, 40%, 10% and 0%.

Something is wrong, but I do not know what. I'll have to consult Stevens.