Getting the correct process id from child processes

You're running into a scoping issue with the variable childProcess. Variables in JavaScript are function scoped. Because of this, the variable isn't exclusive to the loop.

Simplified example of code acting how you have it:

for (var i = 0; i < 3; ++i) {
    var someNumber = i;

    setTimeout(function() {
        console.log(someNumber);
    }, 100);
}

outputs: 3, 3, 3

And the fix:

for (var i = 0; i < 3; ++i) {
    (function() {
        var someNumber = i;

        setTimeout(function() {
            console.log(someNumber);
        }, 100);
    })();
}

You want to wrap your logic in a closure so that the variable childProcess exists with that scope. The above code outputs the expected 0, 1, 2

Looks like a closure problem. Pass the loop variable to a function so that scope is not affected.

var mod_child_process = require('child_process');
var mod_events        = require('events');

for (var i=0; i<3; ++i)
{
    var childProcess;
    childProcess = mod_child_process.spawn('cmd.exe');  
    displayid(childProcess);  
}

function displayid(childProcess){
    childProcess.stdout.on('data', function(data) {         
    console.log('stdout (' + childProcess.pid + '): ' + data);
    });
}

Output is

stdout (368): Microsoft Windows [Version 6.2.9200]
stdout (20268): Microsoft Windows [Version 6.2.9200]
stdout (15728): Microsoft Windows [Version 6.2.9200]

Without it

stdout (10964): Microsoft Windows [Version 6.2.9200]
stdout (10964): Microsoft Windows [Version 6.2.9200]
stdout (10964): Microsoft Windows [Version 6.2.9200]