I don't get any missed numbers; I always get 2..11. I moved the wait_all_children() line above the line that prints a newline so that the output is clearer and more consistent:

2       3       4       5       6       7       9       8       10    
+  11
2       3       4       5       6       7       8       9       10    
+  11
2       3       4       5       6       7       8       9       10    
+  11
2       3       4       5       6       7       8       9       10    
+  11
2       3       4       5       6       7       8       9       10    
+  11
2       3       4       5       6       7       8       9       10    
+  11
2       3       4       5       6       7       8       9       10    
+  11
2       3       4       5       6       7       8       9       10    
+  11
2       3       4       5       6       7       8       9       10    
+  11
2       3       4       5       6       7       8       9       10    
+  11
2       3       4       5       6       7       10      9       8     
+  11
2       3       4       5       6       7       9       10      11    
+  8
2       3       4       5       6       7       9       8       11    
+  10
2       3       4       5       6       7       8       9       10    
+  11
2       3       4       5       6       7       9       11      8     
+  10
2       3       4       5       6       7       8       9       10    
+  11
2       3       4       5       6       7       8       9       10    
+  11
2       3       4       5       6       7       8       9       10    
+  11
2       3       4       5       6       8       9       7       11    
+  10
2       3       4       5       6       7       9       10      8     
+  11
2       3       4       5       6       7       8       9       10    
+  11
2       3       4       5       6       7       8       9       10    
+  11
2       3       4       5       6       7       8       9       10    
+  11
2       3       4       5       6       7       8       9       10    
+  11
2       3       4       5       6       7       8       9       10    
+  11
2       3       4       5       6       8       9       7       11    
+  10
2       3       4       5       6       7       8       9       10    
+  11
2       3       4       5       6       7       8       9       10    
+  11
2       3       4       5       6       7       8       9       10    
+  11
2       3       4       5       6       7       8       9       10    
+  11
2       3       4       5       6       8       9       10      7     
+  11
2       3       4       5       6       7       8       10      9     
+  11
2       3       4       5       6       7       9       10      8     
+  11
2       3       4       5       6       7       8       9       11    
+  10
2       3       4       5       6       7       9       10      8     
+  11
2       3       4       5       6       7       8       9       10    
+  11
2       3       4       5       6       7       8       9       10    
+  11
2       3       4       5       6       7       9       10      8     
+  11
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Doing things in parallel means that you can't guarantee in what order they will finish. Sometimes one process takes a bit longer than the others and so the numbers appear "out of order".